For assessment of gastrointestinal protein loss, seven patients suspected of having protein-losing enteropathy were studied by gamma camera imaging using in-111 transferrin. Four patients with a value equal to or more than 20 ml/day on the alpha-antitrypsin clearance test or a high protein content on the gastric juice showed definite intestinal activity on an early image, demonstrating protein loss. The loss site could be determined by observing the movement of radiotracer within the bowel lumen on the subsequent images. The three patients with a value of less than 20 ml/day in the test showed no obvious intestinal activity throughout the study. Therefore, in-111 transferrin abdominal imaging appears to be useful for determining the loss site as well as for establishing the rapid diagnosis of protein-losing enteropathy.