The serum levels of CA 19-9 were determined in the follow-up of 370 patients with colorectal cancer and compared with CEA. An increase in CA 19-9 preceded clinical diagnosis of recurrence in 25% of 72 patients. The mean time between the rise in CA 19-9 and clinical diagnosis of relapse was 3.7 months (median 3). Sensitivity of CA 19-9 in the early diagnosis of recurrence was much lower than that obtained for CEA (75%). Only 1 patient had elevated CA 19-9 levels and normal CEA.