In the 35-year period, 1950-1984, 154 Olmsted County, Minnesota, infants were diagnosed with definite infantile hypertrophic pyloric stenosis (IHPS). Patients were identified using outpatient and inpatient records of all providers of care to the circumscribed population, and ascertainment was complete insofar as diagnosed cases are concerned. The overall incidence of IHPS was 2.6 per 1000 person-years (95% confidence interval: 2.2-3.0), with a male:female ratio of 4.1:1. A dramatic rise in incidence was seen among male infants over the study period, but not for females, so that by 1980-1984 the rates for the two sexes were 6.2 and 0.9 per 1000 person-years, respectively. Improvements in diagnostic capabilities and case identification may have occurred but seem unlikely to entirely account for these changes. Aetiologic hypotheses should reflect the different trends for male and female infants.