The epidemiology of ulcerative colitis in Stockholm County over a 25-year period, 1955-79, was investigated. Only definite cases in accordance with specified acceptance criteria were included. There were 1274 cases--681 males and 593 females. The proportion of patients with proctitis, left-sided, and total extent of disease of diagnosis remained constant over the study period, as did the time interval between onset of symptoms and definite diagnosis. The peak incidence in relation to age increased slightly but remained in the 3rd and 4th decade through the study period. The incidence in men over 40 years old increased markedly towards the end of the study.