Infantile hypertrophic pyloric stenosis is a relatively common condition and there is some recent evidence that the incidence is increasing in this country. Gastric outlet obstruction is caused by hypertrophy of the pyloric smooth muscle and the clinical presentation is with non-bilious vomiting starting at the age of 3-4 weeks. The diagnosis can usually be made by palpation of the hypertrophied pylorus. Operation has now superseded medical treatment as the treatment of choice but meticulous assessment and correction of fluid and electrolyte imbalance is essential preoperatively. The operation of pyloromyotomy as described by Ramstedt in 1912 is simple and effective, but attention to detail is necessary if minimal morbidity and zero mortality are to be achieved. Untreated, the mortality is high but, after successful treatment, these babies are healthy and normal, so that treatment is both worth while and gratifying. The aetiology is obscure; a polygenic pattern of inheritance has been shown to be a predisposing factor, but the postnatal precipitating factors are less defined. Early hopes that the hormone gastrin might prove to be the key have not been sustained by more recent research, but the role of other gastrointestinal hormones awaits clarification.