A random sample of 4,807 men and women, aged 30, 40, 50, and 60 years, who lived in the western part of Copenhagen County, was drawn from the National Central Person Registry. A total of 226 subjects who were not of Danish origin were omitted. The response rate was 78.8% (3,608/4,581). Each person had his or her gallbladder examined by ultrasonography. The examinations took place between November 1982 and February 1984. The overall prevalence of gallstone disease (cases with stones and cholecystectomized cases) in males aged 30, 40, 50, and 60 years was 1.8%, 1.5%, 6.7%, and 12.9%, respectively. The corresponding prevalence in females was 4.8%, 6.1%, 14.4%, and 22.4%, respectively. Differences according to sex were significant in all age groups. Differences between the 40- and 50-year and 50- and 60-year age groups were significant in both sexes. Among subjects with gallstone disease, the disease was unknown to the proband in the majority of males and in the 30-year-old females, but only in half of the women aged 40, 50, and 60 years. The prevalence of clinically diagnosed gallstones was not significantly different between respondents and nonrespondents.