A cohort of 5,546 ulcerative colitis patients was identified from the Danish Hospital Discharge Register for 1977–1989. Patients not included in the cohort comprised those with proctitis, those treated in outpatient clinics and those for whom follow-up was less than 1 year. The cohort was linked to the Danish Cancer Registry in order to assess the risks for colorectal and other cancers. The linkage revealed a significant increase in the number of colorectal cancers over that in the general population (RR = 1.8; n = 42; 95% CI = 1.3–2.4) with consistent relative risks during early and late follow-up. The relative risk was considerably higher among younger (20–39 years: RR = 22; n = 8; 95% CI = 9.7–44) than older patients (≥60 years: RR = 1.3; n = 25; 95% CI = 0.8–1.9), but the risk difference between patients and the general population was approximately constant across all ages. In addition, we observed a significant increase in the relative risk of hepatobiliary cancers (RR = 2.3; n = 9; 95% = 1.0–4.3) and a slight but significant increase in the relative risk of non-melanoma skin cancer (RR = 1.4; n = 37; 95% CI = 1.0–1.9). In summary, our population-based study confirms the increased risk of colorectal cancer among patients with ulcerative colitis and provides new leads suggesting that hepatobiliary cancer and non-melanoma skin cancer should be considered as possible sites for future patient monitoring. © 1995 Wiley-Liss, Inc.