diff --git a/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/Span.java b/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/Span.java
index 3cbc67531..4a152355d 100644
--- a/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/Span.java
+++ b/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/Span.java
@@ -16,7 +16,6 @@
 
 package org.springframework.cloud.sleuth;
 
-import java.math.BigInteger;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -421,15 +420,30 @@ public class Span {
 	}
 
 	/**
-	 * Represents hex string as long
+	 * Parses a 1 to 32 character lower-hex string with no prefix into an unsigned long, tossing any
+	 * bits higher than 64.
 	 */
 	public static long hexToId(String hexString) {
 		Assert.hasText(hexString, "Can't convert empty hex string to long");
-		try {
-			return new BigInteger(hexString, 16).longValue();
-		} catch (NumberFormatException e) {
-			throw new IllegalArgumentException("Malformed id [" + hexString + "]", e);
+		int length = hexString.length();
+		if (length < 1 || length > 32) throw new IllegalArgumentException("Malformed id");
+
+		// trim off any high bits
+		int i = length > 16 ? length - 16 : 0;
+
+		long result = 0;
+		for (; i < length; i++) {
+			char c = hexString.charAt(i);
+			result <<= 4;
+			if (c >= '0' && c <= '9') {
+				result |= c - '0';
+			} else if (c >= 'a' && c <= 'f') {
+				result |= c - 'a' + 10;
+			} else {
+				throw new IllegalArgumentException("Malformed id");
+			}
 		}
+		return result;
 	}
 
 	@Override
diff --git a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/SpanTests.java b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/SpanTests.java
index 1021a6213..bed930500 100644
--- a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/SpanTests.java
+++ b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/SpanTests.java
@@ -52,6 +52,16 @@ public class SpanTests {
 		then(someLong).isEqualTo(123123L);
 	}
 
+	@Test
+	public void should_convert_lower_64bits_of_hex_string_to_long() throws Exception {
+		String hex128Bits = "463ac35c9f6413ad48485a3953bb6124";
+		String lower64Bits = "48485a3953bb6124";
+
+		long someLong = Span.hexToId(hex128Bits);
+
+		then(someLong).isEqualTo(Span.hexToId(lower64Bits));
+	}
+
 	@Test(expected = IllegalArgumentException.class)
 	public void should_throw_exception_when_null_string_is_to_be_converted_to_long() throws Exception {
 		Span.hexToId(null);
diff --git a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/messaging/TraceChannelInterceptorTests.java b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/messaging/TraceChannelInterceptorTests.java
index 37e173ebc..2ab5c63af 100644
--- a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/messaging/TraceChannelInterceptorTests.java
+++ b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/messaging/TraceChannelInterceptorTests.java
@@ -275,6 +275,20 @@ public class TraceChannelInterceptorTests implements MessageHandler {
 		then(TestSpanContextHolder.getCurrentSpan()).isNull();
 	}
 
+	@Test
+	public void downgrades128bitIdsByDroppingHighBits() {
+		String hex128Bits = "463ac35c9f6413ad48485a3953bb6124";
+		String lower64Bits = "48485a3953bb6124";
+		this.tracedChannel.send(MessageBuilder.withPayload("hi")
+				.setHeader(Span.TRACE_ID_NAME, hex128Bits)
+				.setHeader(Span.SPAN_ID_NAME, Span.idToHex(20L)).build());
+		then(this.message).isNotNull();
+
+		long traceId = Span.hexToId(this.message.getHeaders()
+				.get(TraceMessageHeaders.TRACE_ID_NAME, String.class));
+		then(traceId).isEqualTo(Span.hexToId(lower64Bits));
+	}
+
 	@Configuration
 	@EnableAutoConfiguration
 	static class App {
diff --git a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/HttpServletRequestExtractorTests.java b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/HttpServletRequestExtractorTests.java
index fc660f2ad..fe1afc5df 100644
--- a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/HttpServletRequestExtractorTests.java
+++ b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/HttpServletRequestExtractorTests.java
@@ -93,4 +93,19 @@ public class HttpServletRequestExtractorTests {
 			then(e).hasMessageContaining("Malformed id");
 		}
 	}
-}
\ No newline at end of file
+
+	@Test
+	public void should_downgrade_128bit_trace_id_by_dropping_high_bits() {
+		String hex128Bits = "463ac35c9f6413ad48485a3953bb6124";
+		String lower64Bits = "48485a3953bb6124";
+
+		BDDMockito.given(this.request.getHeader(Span.TRACE_ID_NAME))
+				.willReturn(hex128Bits);
+		BDDMockito.given(this.request.getHeader(Span.SPAN_ID_NAME))
+				.willReturn(lower64Bits);
+
+		Span span = this.extractor.joinTrace(this.request);
+
+		then(span.getTraceId()).isEqualTo(Span.hexToId(lower64Bits));
+	}
+}