The diagnosis of hypertrophic pyloric stenosis (HPS) with ultrasonography (US) is dependent on measurements of pyloric diameter (PD), pyloric length (PL), and muscle thickness. The authors were unable to confidently diagnose the condition with US in 45% of patients who underwent surgery for HPS because all three criteria were not fulfilled. An overall measurement of the "amount" of pyloric hypertrophy was introduced: pyloric volume (PV), which was equated to 1/4 pi x PD2 x PL. No overlap was found between patients with HPS (n = 22; PV = 3.13 mL; range, 1.4-5.1 mL) and asymptomatic control subjects (n = 28; PV = 0.65 mL; range, 0.2-1.3 mL) or symptomatic subjects without HPS (n = 25; PV = 0.86 mL; range, 0.4-1.3 mL). A positive correlation was found between age at diagnosis and PV, a finding reflecting that HPS is an acquired condition. In patients less than 4 weeks of age, the criterion of PV greater than or equal to 1.4 mL proved to aid in the identification of early HPS more accurately than any existing criteria.