We examined the relation of carcinoembryonic antigen levels to time, site and extent of recurrence in 358 patients with colorectal cancer. The recurrence rate was higher in patients with Dukes' B and Dukes' C lesions who had preoperative levels higher than 5 ng per milliliter. There was a linear inverse correlation between preoperative levels and estimated mean time to recurrence in patients with Dukes' B and C lesions, ranging from 30 months for a level of 2 to 9.8 months for a level of 70 ng per milliliter. In patients with Dukes' C lesions the median time to recurrence was 13 months if preoperative levels were higher than 5 ng per milliliter, and 28 months if they were lower. Preoperative carcinoembryonic antigen levels in patients with resectable Dukes' B and C cancer provided an additional criterion for allocating these patients to groups at high or low risk for recurrence.