There are no adequate studies of the incidence of urolithiasis in the United States, in spite of earlier claims that a "stone belt" exists in the southeastern section of the country. This report is the first description of the incidence and recurrence rates for symptomatic noninfected renal stones in a well-defined population. A total of 798 patients were enrolled in the study group, of whom 672 were incidence cases having had their first episode as documented residents of Rochester, Minnesota, between 1950 and the end of 1974. The annual age-adjusted incidence rate for females was stable over the 25-year study period at 36.0 per 100,000 population. That for males increased significantly (P less than 0.02) from 78.5 per 100,000 to 123.6 per 100,000. Recurrence calculations showed a high rate for both sexes in the first year, followed by lower but constant rates for all succeeding years.