Two series of young patients (less than 40 years of age) with colorectal carcinoma (22 idiopathic carcinomas and 25 carcinomas complicating ulcerative proctocolitis), well matched for age and sex, were compared with regard to clinical features, tumour morphology and stage, and ultimate outcome after surgery. The cure rate in both series was low. Although a failure to diagnose colitis carcinoma accurately at an early stage might have contributed to the poor results, such a delay could hardly be responsible for the bad prognosis in patients with idiopathic carcinoma. The vast majority of the patients in both groups studied had highly malignant and/or mucoid adenocarcinoma, and surgery was palliative in about 40% of the patients in both series, owing to widespread dissemination. The general impression gained from this study of factors of histologic grade of malignancy, extent of spread, and survival rate was that colorectal carcinomas in the young, irrespective of being idiopathic or complicating ulcerative colitis, run a rapid course and have a gloomy prognosis. The outloook depends largely on the biologic characteristics of the tumours concerned. The results support previous statements that prophylactic surgery is justified in patients with long-standing ulcerative colitis with total involvement of the colon, particularly in the young. Regrettably, patients with idiopathic carcinoma will not have this chance.