diff --git a/README.md b/README.md
index f7457b9..4c1ebc1 100644
--- a/README.md
+++ b/README.md
@@ -268,6 +268,11 @@ event loop is about to drain. Spans that are still buffered when `process.exit()
 is called explicitly are lost, because explicit exits do not run the exit-time
 flush.
 
+A span keeps at most 128 distinct attributes, the OpenTelemetry SDK's default
+`attributeCountLimit`. Writes to a key the span already has always land. A new
+key past the limit is dropped and counted, and the count is exported with the
+span as the `droppedAttributesCount` OTLP field.
+
 ## License
 
 [MIT](./LICENSE)
diff --git a/lib/flush.js b/lib/flush.js
index 46e84c4..39c06cf 100644
--- a/lib/flush.js
+++ b/lib/flush.js
@@ -140,6 +140,12 @@ function spanToOtlp(span) {
 
   if (attributes.length > 0) otlpSpan.attributes = attributes;
 
+  // The OTLP field is dropped_attributes_count, spelled
+  // droppedAttributesCount in ProtoJSON.
+  const droppedAttributesCount = span.droppedAttributesCount;
+  if (droppedAttributesCount > 0)
+    otlpSpan.droppedAttributesCount = droppedAttributesCount;
+
   const status = span.status;
   if (status.code !== 0 || status.message) otlpSpan.status = status;
 
diff --git a/lib/span.js b/lib/span.js
index fdf76c8..cfbc16b 100644
--- a/lib/span.js
+++ b/lib/span.js
@@ -30,6 +30,11 @@ const kAllZero16 = '0000000000000000';
 const kTraceFlagSampled = 0x01;
 const kTraceFlagRandom = 0x02;
 
+// A span keeps at most this many distinct attribute keys, the OpenTelemetry
+// SDK's default attributeCountLimit. New keys past it are dropped and
+// counted, while writes to keys the span already has always land.
+const kAttributeCountLimit = 128;
+
 // Two hex digits for every trace flags byte, so building a traceparent
 // never formats a number.
 const kTraceFlagsHex = Array.from({ length: 256 }, (_, i) =>
@@ -100,6 +105,12 @@ class Span {
   #startMonotonic;
 
   #attributes;
+  // #attributeLimit counts setAttribute calls until they reach the cap.
+  // Distinct keys never outnumber writes, so spans below the cap need no
+  // per-key accounting. Once writes reach the cap, it holds the exact
+  // distinct count and the dropped count instead, so an ordinary span
+  // initializes one field for all of this.
+  #attributeLimit = 0;
   #events;
   #status;
   #traceFlags;
@@ -155,7 +166,40 @@ class Span {
   }
 
   setAttribute(key, value) {
-    this.#attributes[key] = value;
+    const writes = this.#attributeLimit;
+    if (typeof writes === 'number' && writes < kAttributeCountLimit) {
+      this.#attributeLimit = writes + 1;
+      this.#attributes[key] = value;
+      return this;
+    }
+    return this.#setAttributeCapped(key, value);
+  }
+
+  // This runs only once writes reach the cap. A key the span does not
+  // have yet holds undefined, and undefined values are never exported, so
+  // a key counts against the cap only while it holds another value. Writing
+  // undefined to a key frees its slot.
+  #setAttributeCapped(key, value) {
+    const attrs = this.#attributes;
+    let limit = this.#attributeLimit;
+    if (typeof limit === 'number') {
+      let count = 0;
+      for (const k of Object.keys(attrs)) if (attrs[k] !== undefined) count++;
+      limit = { count, dropped: 0 };
+      this.#attributeLimit = limit;
+    }
+    const had = attrs[key] !== undefined;
+    const has = value !== undefined;
+    if (has && !had) {
+      if (limit.count === kAttributeCountLimit) {
+        limit.dropped++;
+        return this;
+      }
+      limit.count++;
+    } else if (had && !has) {
+      limit.count--;
+    }
+    attrs[key] = value;
     return this;
   }
 
@@ -163,6 +207,11 @@ class Span {
     return this.#attributes;
   }
 
+  get droppedAttributesCount() {
+    const limit = this.#attributeLimit;
+    return typeof limit === 'number' ? 0 : limit.dropped;
+  }
+
   addEvent(name, attributes) {
     // Only the internal exception producer uses events today; cap them
     // once addEvent becomes part of a public API.
diff --git a/test/test-attribute-limits.js b/test/test-attribute-limits.js
new file mode 100644
index 0000000..167ee6d
--- /dev/null
+++ b/test/test-attribute-limits.js
@@ -0,0 +1,105 @@
+'use strict';
+
+const assert = require('node:assert');
+const { after, before, describe, it } = require('node:test');
+
+const { Span, SPAN_KIND_INTERNAL } = require('../lib/span');
+const { startTracing, flushAndCollect, spanAttrs } = require('./common/otel');
+
+const kAttributeCountLimit = 128;
+
+describe('span attribute limits', () => {
+  let collector;
+
+  before(async () => {
+    collector = await startTracing();
+  });
+
+  after(() => collector.close());
+
+  it('caps distinct keys at 128 and exports the dropped count', async () => {
+    const span = new Span('attribute-cap', SPAN_KIND_INTERNAL);
+    for (let i = 0; i < 200; i++) span.setAttribute(`key${i}`, i);
+    span.end();
+
+    const [exported] = await flushAndCollect(collector);
+    const attrs = spanAttrs(exported);
+
+    assert.strictEqual(exported.attributes.length, kAttributeCountLimit);
+    assert.partialDeepStrictEqual(exported, { droppedAttributesCount: 72 });
+    // The first 128 keys land and the rest are dropped.
+    assert.strictEqual(attrs.key0, '0');
+    assert.strictEqual(attrs.key127, '127');
+    assert.ok(!('key128' in attrs));
+    assert.ok(!('key199' in attrs));
+  });
+
+  it('lets writes to existing keys land past the cap', async () => {
+    const span = new Span('existing-key', SPAN_KIND_INTERNAL);
+    for (let i = 0; i < kAttributeCountLimit; i++)
+      span.setAttribute(`key${i}`, i);
+    span.setAttribute('key0', 'updated');
+    span.setAttribute('past-the-cap', 'dropped');
+    span.end();
+
+    const [exported] = await flushAndCollect(collector);
+    const attrs = spanAttrs(exported);
+
+    assert.strictEqual(exported.attributes.length, kAttributeCountLimit);
+    assert.strictEqual(attrs.key0, 'updated');
+    assert.ok(!('past-the-cap' in attrs));
+    assert.partialDeepStrictEqual(exported, { droppedAttributesCount: 1 });
+  });
+
+  it('frees a slot when a key is cleared past the cap', async () => {
+    const span = new Span('cleared-key', SPAN_KIND_INTERNAL);
+    for (let i = 0; i < kAttributeCountLimit; i++)
+      span.setAttribute(`key${i}`, i);
+    // Clearing a key frees its slot for the same key or a new one.
+    span.setAttribute('key0', undefined);
+    span.setAttribute('key0', 'back');
+    span.setAttribute('key1', undefined);
+    span.setAttribute('other', 'lands');
+    span.end();
+
+    const [exported] = await flushAndCollect(collector);
+    const attrs = spanAttrs(exported);
+
+    assert.strictEqual(exported.attributes.length, kAttributeCountLimit);
+    assert.strictEqual(attrs.key0, 'back');
+    assert.strictEqual(attrs.other, 'lands');
+    assert.ok(!('key1' in attrs));
+    assert.ok(!Object.hasOwn(exported, 'droppedAttributesCount'));
+  });
+
+  it('counts keys exactly after many writes to one key', async () => {
+    // The writes alone reach the cap while the span has one key, so the
+    // exact count starts there. A null value holds a slot like any other.
+    const span = new Span('rewrites', SPAN_KIND_INTERNAL);
+    for (let i = 0; i < kAttributeCountLimit; i++) span.setAttribute('same', i);
+    for (let i = 1; i < kAttributeCountLimit - 1; i++)
+      span.setAttribute(`key${i}`, i);
+    span.setAttribute('nullish', null);
+    span.setAttribute('one-too-many', 'dropped');
+    span.end();
+
+    const [exported] = await flushAndCollect(collector);
+    const attrs = spanAttrs(exported);
+
+    assert.strictEqual(exported.attributes.length, kAttributeCountLimit);
+    assert.strictEqual(attrs.same, String(kAttributeCountLimit - 1));
+    assert.ok('nullish' in attrs);
+    assert.ok(!('one-too-many' in attrs));
+    assert.partialDeepStrictEqual(exported, { droppedAttributesCount: 1 });
+  });
+
+  it('exports no droppedAttributesCount under the cap', async () => {
+    const span = new Span('under-the-cap', SPAN_KIND_INTERNAL);
+    span.setAttribute('http.request.method', 'GET');
+    span.end();
+
+    const [exported] = await flushAndCollect(collector);
+
+    assert.ok(!Object.hasOwn(exported, 'droppedAttributesCount'));
+  });
+});