The files of the Oxford Record Linkage Study were employed to identify 220 infants presenting with infantile hypertrophic pyloric stenosis (IHPS) in the 6-year period 1966 to 1971. Information on these infants was obtained from birth certificates and maternity notes. The overall incidence was 2.5 per 1000 livebirths. There was a distinct seasonal variation, with highest incidence to infants born in the third quarter of the year as well as variation in incidence with area: the cities had much lower rates of IHPS than the adjacent rural or small urban areas. It was shown that the rates in the south and east of the area studied were far greater than in the north and west. In the present study there was no excess of primiparae, the peak maternal age group was 20 to 24; there was a slight excess of parents of social classes I and II; and a significant association with mothers who were Rhesus negative. The rate of IHPS among sibs was 85 per 1000. Though there was the usual correlation with the male sex (M:F ratio = 5.5:1), there was no variation with birthweight and only among the females was an association found with prolonged gestation. There appeared to be an inverse relation between gestation and age on admission to hospital.