Abstract During the period 1943–67, 903 Danish patients aged <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.