The pyloric region of 58 infants who presented with vomiting was examined by real-time ultrasound. Transverse diameter of the pylorus and single wall thickness were measured. In 18 patients sonographic diagnosis of hypertrophic pyloric stenosis (HPS) was made and subsequently confirmed by surgery. The mean (+/- one standard deviation) of the transverse diameter and of the pyloric wall thickness in patients with HPS was 15 (+/- 1) mm. and 5.5 (+/- 0.7) mm., respectively. Forty infants matched for age and sex distribution without gastrointestinal symptoms served as controls. The measurements for the transverse diameter were 9.6 (+/- 1.8) mm. and for the wall thickness 1.9 (+/- 0.5) mm. Diameter values of patients with HPS were significantly (p less than 0.001) higher than those of the control group and those of the remaining 40 patients without HPS. Real-time sonography proved a reliable tool in the diagnosis of HPS.