From January 1986 to February 1994, 198 children were operated on for hypertrophic pyloric stenosis (HPS). Postoperative follow-up have been carried out in 194 cases. The children were divided into two groups: group A (n = 134; 69.1%): without any postoperative diet troubles (n = 52) or simple regurgitations (n = 82), and group B (n = 60; 30.9%) presenting more significant vomiting requiring medical treatment (n = 52) or a prolongation of parental nutrition (n = 8). A retrospective study of the different factors which can possibly explain this postoperative vomiting was carried out. The criteria having an influence are: the age (44.5 days in group A; 35.7 days in group B; (p < 0.001) the weight at the time of the operation (3921) g in group A; 3647 in group B; p = 0.01) the thickness of the pylorus at the pre-operative ultrasound scan (5.2 mm in group A; 47 in group B; p < 0.015). The other studied criteria (prematurity, birth weight, delay in diagnosis, weight loss, hydroelectrolytic abnormalities, surgical approach way-subcostal or umbilical-, surgical difficulties and operation duration) are not statistically significant. The young age (and therefore the low weight) at the time of the pyloromyotomy can easily explain the post-operative vomiting through the physiological immaturity of the lower sphincter of the esophagus. It is more paradoxical to note that these difficulties are all the more frequent because the pyloric tumor is less thick at the ultrasound scan. But this criterion is also directly related to the child's age (average thickness of 4.5 mm before the age of one month and 5.8 mm after the age of two months; p < 0.0001). These data suggest the importance of systematic medical treatment to prevent postoperative vomiting in high-risk children, in order to decrease hospital stay (4.14 days in group A; 5.20 days in group B; p < 0.0001).