The diagnostic accuracy of ultrasonography and gastrointestinal endoscopy was compared in 63 infants who were operated on for infantile hypertrophic pyloric stenosis. Endoscopy was far more accurate than ultrasonography, the diagnosis being made in 97 and 81% of the cases, respectively. The difference between the techniques was even more obvious in the younger patients and in those with a short history of vomiting. The ability to detect coexistent or other causes of vomiting with endoscopy appeared advantageous. The endoscopic procedure is easily done without general anesthesia and was without complications in our series. We recommend endoscopy as an important tool in very young patients with few clinical signs other than vomiting, allowing for appropriate treatment without delay.