In a randomized prospective clinical trial in 128 patients undergoing radical surgical resection of primary colorectal cancer, 63 patients received intravenous fluorouracil (5-FU) in two courses, four and eight weeks after surgery; 65 controls received no chemotherapy. The duration of follow-up exceeded five years in all cases, and 28 patients have died in each group. Recurrent disease was present in 26 5-FU patients (41.2 percent) and in 22 controls (33.9 percent). There is no evidence that short-term 5-FU therapy was of significant value in the chemoprophylaxis of colorectal cancer.