diff --git a/independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/ResteasyReactiveOutputStream.java b/independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/ResteasyReactiveOutputStream.java
index 90d13ba225ec0..d5cf25c8240fd 100644
--- a/independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/ResteasyReactiveOutputStream.java
+++ b/independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/ResteasyReactiveOutputStream.java
@@ -6,12 +6,15 @@
 import io.vertx.core.Context;
 import io.vertx.core.Handler;
 import io.vertx.core.buffer.Buffer;
+import io.vertx.core.http.HttpHeaders;
 import io.vertx.core.http.HttpServerRequest;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InterruptedIOException;
 import java.io.OutputStream;
+import javax.ws.rs.core.MultivaluedMap;
 import org.jboss.logging.Logger;
+import org.jboss.resteasy.reactive.server.core.LazyResponse;
 import org.jboss.resteasy.reactive.server.core.ResteasyReactiveRequestContext;
 
 public class ResteasyReactiveOutputStream extends OutputStream {
@@ -233,12 +236,24 @@ private void prepareWrite(ByteBuf buffer, boolean finished) throws IOException {
                 } else {
                     context.serverResponse().setResponseHeader(HttpHeaderNames.CONTENT_LENGTH, "" + buffer.readableBytes());
                 }
-            } else if (!request.response().headers().contains(HttpHeaderNames.CONTENT_LENGTH)) {
+            } else if (!contentLengthSet()) {
                 request.response().setChunked(true);
             }
         }
     }
 
+    private boolean contentLengthSet() {
+        if (request.response().headers().contains(HttpHeaderNames.CONTENT_LENGTH)) {
+            return true;
+        }
+        LazyResponse lazyResponse = context.getResponse();
+        if (!lazyResponse.isCreated()) {
+            return false;
+        }
+        MultivaluedMap<String, Object> responseHeaders = lazyResponse.get().getHeaders();
+        return (responseHeaders != null) && responseHeaders.containsKey(HttpHeaders.CONTENT_LENGTH);
+    }
+
     /**
      * {@inheritDoc}
      */
diff --git a/independent-projects/resteasy-reactive/server/vertx/src/test/java/org/jboss/resteasy/reactive/server/vertx/test/response/ContentLengthTest.java b/independent-projects/resteasy-reactive/server/vertx/src/test/java/org/jboss/resteasy/reactive/server/vertx/test/response/ContentLengthTest.java
new file mode 100644
index 0000000000000..2faf6d56d628e
--- /dev/null
+++ b/independent-projects/resteasy-reactive/server/vertx/src/test/java/org/jboss/resteasy/reactive/server/vertx/test/response/ContentLengthTest.java
@@ -0,0 +1,56 @@
+package org.jboss.resteasy.reactive.server.vertx.test.response;
+
+import static io.restassured.RestAssured.when;
+import static org.hamcrest.Matchers.greaterThan;
+import static org.hamcrest.core.IsNull.nullValue;
+
+import java.io.ByteArrayInputStream;
+import java.nio.charset.StandardCharsets;
+import java.util.Collections;
+import java.util.UUID;
+import javax.ws.rs.GET;
+import javax.ws.rs.Path;
+import javax.ws.rs.Produces;
+import javax.ws.rs.core.HttpHeaders;
+import javax.ws.rs.core.MediaType;
+import javax.ws.rs.core.Response;
+import org.jboss.resteasy.reactive.server.vertx.test.framework.ResteasyReactiveUnitTest;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+public class ContentLengthTest {
+
+    private static final int NUMBER_OF_COPIES = 500;
+
+    @RegisterExtension
+    static ResteasyReactiveUnitTest runner = new ResteasyReactiveUnitTest()
+            .withApplicationRoot((jar) -> jar
+                    .addClasses(FileResource.class));
+
+    @Test
+    void testResponseHeaders() {
+        when()
+                .get("/file")
+                .then()
+                .statusCode(200)
+                .header(HttpHeaders.CONTENT_LENGTH,
+                        greaterThan(
+                                "" + (NUMBER_OF_COPIES * UUID.randomUUID().toString().getBytes(StandardCharsets.UTF_8).length)))
+                .header("transfer-encoding", nullValue());
+    }
+
+    @Path("/file")
+    public static class FileResource {
+
+        @GET
+        @Produces(MediaType.TEXT_PLAIN)
+        public Response hello() {
+            byte[] bytes = String.join(";", Collections.nCopies(NUMBER_OF_COPIES, UUID.randomUUID().toString()))
+                    .getBytes(StandardCharsets.UTF_8);
+            return Response.ok(new ByteArrayInputStream(bytes), "text/plain")
+                    .header(HttpHeaders.CONTENT_LENGTH, bytes.length)
+                    .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename = " + "uuid.txt")
+                    .build();
+        }
+    }
+}