Twenty children with suspected hypertrophic pyloric stenosis were studied by sonography. The thickened pyloric muscle is seen as an anechoic mass 1.5 cm or more in diameter, with strong central echoes. Of the 16 infants with a pyloric mass of 1.5 cm or more, 15 underwent surgery and the diagnosis was confirmed. In four patients with a pylorus less than 1.5 cm, the diagnosis of pyloric stenosis was excluded and all were discharged without surgery. Real-time scanning is useful in localizing the pylorus and in demonstrating lack of passage of gastric content across the narrowed pyloric canal.