All patients with ulcerative colitis referred to Rigshospitalet, Copenhagen, from 1 April 1964 to 1 January 1983 (18 years and 9 months) were studied from time of referral until death, proctocolectomy, or end of the study (1983). There were 759 patients, 423 females (56%) and 336 males (44%). None was lost to follow-up study. Median time from onset of disease until death, proctocolectomy, or end of the study was 11 years (range, 0-54 years). Median age at onset was 28 years (range, 0-83) among the males and 28 years (range, 4-83) among the females. Pancolitis was present in 312 patients (41%), left-sided colitis in 212 (28%), and haemorrhagic proctitis in 235 (31%). Surgical treatment was performed in 299 patients (39%): proctocolectomy in 197 (26%), colectomy with occluded rectal stump in 72 (9%), and colectomy with ileorectal anastomosis in 30 (4%). Altogether, 49 patients developed cancer, 20 being intestinal and 29 extraintestinal cancer. Compared with the general population matched for age, sex, and calendar time, there was an excessive number with intestinal cancer in both sexes (p less than 0.05). In females the number with extraintestinal cancer was higher than in the general population (p less than 0.01), a finding that has not been reported elsewhere. We found a similar, significantly increased incidence of extraintestinal cancer in females with Crohn's disease in a previous report. We found no increased risk of colorectal cancer in patients with early onset of ulcerative colitis. For all age classes we found that the age of appearance of colorectal cancer followed the equation: age at colorectal cancer = 14 + age at onset of ulcerative colitis. We found no higher potential for development of colorectal cancer in patients with pancolitis. In our series the incidence of colorectal cancer in pancolitis and left-sided colitis was equal. The incidence in patients with haemorrhagic proctitis was zero.