The cause of hypertrophic pyloric stenosis is unknown. The number of theories about its cause, most of them vague and none accepted generally, attest to this. Our interest in the problem of the etiologic basis of this disease was aroused by seeing an adult with active peptic ulcer, who had undergone a Rammstedt operation as an infant. Since both hypertrophic pyloric stenosis and duodenal ulcer may be related to autonomic imbalance, the idea suggested itself that if we could analyze a number of similar cases, studying the patients as well as their families, we might learn about factors predisposing to this disease; in other words, if an autonomic imbalance exists in an infant with hypertrophic pyloric stenosis it may manifest itself later in life in one or another form. Our data on patients and their families were obtained by direct contact with the patients or their physicians and by a