This study was undertaken to clarify the source of blood in the vomitus of patients with hypertrophic pyloric stenosis (HPS). Twenty-one infants with HPS were examined. Hematemesis was noted in 14 infants. Esophagogastric endoscopy showed a 100% incidence of esophagitis and in one patient gastric erosion was also observed. Histological study of the esophageal mucosa showed evidence of esophagitis in 18 patients (85.7%). Preoperative pH monitoring showed gastroesophageal reflux (GER) in all infants. Excessive acid exposure (> or = 7%) was significantly correlated with the grade of esophagitis and the incidence of hematemesis, whereas acid exposure time was shorter in the cases without histological esophagitis. These results suggested that the source of bleeding in HPS is the esophageal mucosa affected by esophagitis secondary to excessive acid reflux. Although there is obvious massive gastroesophageal reflux in HPS, it is too difficult to evaluate the lower esophageal sphincter function in HPS.