During the period 1943-67, 903 Danish patients aged less than 40 years had colorectal carcinoma. The patients were followed up for up to 41 years and during this period 44 of 501 (9 per cent) operated on for cure developed a metachronous colorectal carcinoma. The cumulative risk of a metachronous colorectal carcinoma was 30 per cent after up to 41 years of observation. The occurrence of a metachronous colorectal carcinoma was evenly distributed in the observation period. The cumulative survival rate after operation for a metachronous colorectal carcinoma was 41 per cent after 20 years of observation. We propose a lifelong follow-up programme after resection of colorectal carcinoma for cure in this age group, including annual Hemoccult test and colonoscopy at 3-year intervals.