Data on 818 patients who had undergone curative resection for Dukes' B2 or Dukes' C carcinoma of the colon and rectum were analyzed to determine the timing and patterns of recurrence based on such tumor characteristics as location, Dukes' stage, grade, ploidy and the presence of obstruction, perforation or adherence to adjacent organs or tissues. Three hundred and fifty-three patients (43 per cent) had recurrent disease. There was recurrence in 52 per cent of patients with carcinoma of the rectum and in 40 per cent of patients with carcinoma of the colon. The median time to recurrence for all patients was 16.7 months, with a range from 1 month to 7.5 years. Dukes' C lesions and the presence of adhesion or invasion, or both, or perforation were associated with significantly earlier recurrence. Among patients with recurrence, the most frequent sites were hepatic in 33 per cent, pulmonary in 22 per cent, local or regional, or both, in 21 per cent, intra-abdominal in 18 per cent, retroperitoneal in 10 per cent and peripheral lymph nodes in 4 per cent. Rectal primary sites, when compared with colonic, had proportionally more local or regional, or both, recurrences (p = 0.00003) and fewer involving retroperitoneal nodes (p = 0.022). Both primaries of the rectum and colon at stage C, when compared with stage B, had fewer local or regional recurrences, or both (p = 0.01), but a greater tendency to involve retroperitoneal or peripheral nodes. Primaries of the colon with adhesion to, or invasion of, adjacent organs had a lesser tendency to pulmonary metastasis (p = 0.036). Whereas the grade of anaplasia and ploidy had a strong influence on the rate of recurrence, they did not influence timing or patterns of recurrence. Patterns of recurrence based on the characteristics of the tumor may facilitate selection of the most appropriate adjuvant procedures, particularly those directed toward local or regional recurrence, or both, and also may guide efforts at early recognition of recurrence.