In a nationwide cohort of gallstone patients we evaluated the hypothesis that gall stones are associated with an increased risk of cancer of the colon. In the Danish Hospital Discharge Register we identified 42,098 patients with gallstones diagnosed in 1977-1989. These patients were linked to the Danish Cancer Registry in order to assess their risks of colorectal and other cancers during follow-up to the end of 1992. The analysis showed a modest increase in the number of cancers at all sites combined (N = 3940; RR = 1.07; 95% CI = 1.0-1.1). A weak association was found for cancer of the colon (N = 360; RR = 1.09; 95% CI = 1.0-1.2), which remained unchanged when analysed by sex, anatomical subsite and duration of follow-up. Multivariate analysis with adjustment for cholecystectomy and clinically defined obesity did not change these estimates to any significant extent. A nonsignificantly increased risk of breast cancer was seen in women five years after initial discharge for gallstones.