Of 227 patients with stage B or C colorectal carcinoma operated for "cure," 132 had a febrile postoperative course. Of the latter, five patients (3.7 per cent) died of sepsis. The five-year actuarial recurrence rate for 227 patients was 53 per cent. When the patients were divided into groups according to stage of disease and postoperative fever, the following was found: Eighty-one low stage patients (B1 + B2) had a 34 per cent five-year actuarial recurrence rate, and 146 high-stage patients (B3 + C4 + C2)-- a 71 per cent rate (P less than 0.0005). Fever occurred postoperatively in 46 per cent of low-stage patients and in 65 per cent of high-stage patients (P = 0.004). In low-stage patients, the five-year actuarial recurrence rate was 3 per cent in the group with an afebrile postoperative course, and 66 per cent in that with fever (P much less than 0.0005). Similarly, in high-stage patients, the recurrence rate was 24 and 93 per cent, respectively in the group with afebrile and febrile postoperative courses (P much less than 0.0005). Preoperative plasma CEA levels seemed to have no bearing upon prognosis, unless above 20 ng/ml. Eighty-two per cent of patients who had serial postoperative plasma CEA measurements and recurrence of cancer had increasing CEA levels. Thus, postoperative fever lasting two or more days was the most unfavorable prognostic factor, highly significant statistically, whereas stage of disease ranked only second in isolating better prognoses among operated patients from those at higher risks of recurrence.