Ultrasound diagnosis of pyloric stenosis depends on the typical "target" and "cervix" pictures, respectively in transverse and longitudinal scan. Furthermore, pyloric dimensions can be considered pathologic when the muscular wall is greater than or equal to 4 mm. in width, having a maximum diameter greater than or equal to 5 mm. and the pyloric canal is greater than or equal to 18 mm in length. At the Department of Paediatric Surgery of Parma, from 1986 to 1988, 34 infants were studied by ultrasound, basing on clinical suspect of pyloric stenosis. 22 of them were subsequently operated on. Pyloric ultrasound was diagnostic in 20 cases, negative in 1 and doubtful in 1, confirming its reliability in 90-95% of cases. No false positive findings were recorded. Basing on these results, the Authors believe that ultrasound must be the first choice in case of clinical suspect of pyloric stenosis in infancy, because of its high diagnostic value and safety. X-ray studies should be indicated only in doubtful cases.