A prospective controlled randomized trial testing adjuvant postoperative combination chemotherapy (5-fluorouracil, lomustine (CCNU) and vincristine) versus no adjuvant therapy in patients operated on for Dukes' C colorectal cancer is reported. In total 334 patients aged less than 70 years were recruited: 205 patients with colonic and 99 with rectal cancer, but there were three protocol violations and these cases are excluded from further consideration. Twenty-seven patients had a limited resection of their cancer. After 5 years' follow-up there was no significant difference in the tumour-free survival rate or in the survival rate between the treated and control groups. Twenty-nine of the 147 patients who started chemotherapy discontinued this treatment because of side-effects, mainly from the gastrointestinal tract. In 30 patients treatment was discontinued because of recurrent disease. The conclusion is that systemic administration of combination chemotherapy for colorectal cancer after operation is not worthwhile in routine clinical practice.