Existing data on the epidemiology of Paget's disease of bone are limited by the lack of directly determined secular trends in clinically diagnosed Paget's disease. In the current study, we examine trends in Paget's disease incidence in Olmsted County, MN, using data from the Rochester Epidemiology Project medical records linkage system. During the period 1950 through 1994, 236 Olmsted County, MN residents were diagnosed for the first time with Paget's disease of bone at a mean age of 69.6 years. Overall, there were 129 (54.7%) men and 107 women, and the age-adjusted incidence of Paget's disease was 12.7 per 100,000 person-years (95% CI 10.4-14.9) among the men compared with 7.0 per 100,000 person-years (95% CI 5.6-8.3) among Olmsted County women (male/female ratio of 1.8:1). The higher incidence in males compared with females and the increase in incidence with older age were statistically significant. The incidence of Paget's disease in Olmsted County seems to have increased over the first part of the study period and then declined. This may have resulted from ascertainment bias: the introduction of a 12-test automated serum chemistry panel in 1974 might have led to a sudden increase in the apparent incidence of Paget's disease followed by a compensatory decrease. In addition, there was a decrease in the proportion of patients who were symptomatic at diagnosis, from 36% in 1950-1959 to 27% in 1980-1994. This finding also suggests that routine measurement of alkaline phosphatase may have led to more diagnosis of asymptomatic individuals. The subsequent fall in the incidence of Paget's disease is consistent with previous reports, although this apparent decline could be artifactual to the extent that the reservoir of undiagnosed cases in the population was exhausted by earlier testing.