The frequency of metachronous colorectal tumours in a prospective 20-year series of patients with colorectal cancer is reported. Thirty-one patients, that is 2.1% of the patients operated on for cure, had metachronous carcinomas, and 5.4% had metachronous polyps. One-third of the metachronous carcinomas were diagnosed at scheduled postoperative control examinations, and the others because of their symptoms or at autopsy. The reported metachronous carcinomas are considered to represent failures of follow-up only and not to indicate their true incidence, since their natural course has been arrested by removal of adenomas whenever found. Several of the metachronous tumours were judged to be overlooked synchronous ones, and therefore a thorough postoperative examination of the remaining large bowel is advocated. The finding of subsequent tumours in this series indicates that continuous follow-up is worthwhile.