&NA;Plasma CEA levels were determined in 280 patients with histologically proven colorectal carcinoma: in 180 once or twice preoperatively and repeatedly postoperatively, and in 100 postoperatively only. Preoperative levels correlated directly with surgical‐pathologic state and inversely with the time of recurrence;i.e., groups of patients with more advanced stage showed higher proportion of elevated CEA levels, and patients with higher levels had recurrences earlier. Levels above 20 ng/ml were strongly suggestive of liver metastases or disseminated disease.Postoperative plasma CEA levels displayed three patterns of variation. 1) Levels fluctuating within normal range, below 3.2 ng/ml, strongly evidenced that surgery was effective, and 2) those fluctuating below 7.5 ng/ml indicated usually nonmalignant concomitant disease; in either group no more than 10 per cent of the patients had recurrences. 3) Levels rising persistently from a postoperative nadir indicated recurrence or, when the rise was dramatic, liver metastases or disseminated cancer in at least 97 per cent of the patients.