It has been reported that lung cancer patients often have raised carcinoembryonic antigen (CEA) levels but the significance of this in diagnosis and follow-up has yet to be established. The results of 256 preoperative investigations in patients with lung cancer are reported. Sequential values after radical surgery and chemotherapy and immunotherapy have been performed in 57 patients during treatment and outpatient follow-up. Ninety-nine per cent of preoperative values were more than 5 ng/ml and 41% greater than 15 ng/ml. Only 6% reached diagnostic levels for malignancy (greater than 52ng/ml) and adenocarcinomas formed 47% (7 out of 15) of these. Sequential estimation in patients during and after treatment showed fluctuations which were related to disease status in 7 (32%) of 22 who have developed secondary disease. In three patients levels of greater than 50 ng/ml preceded clinical evidence of recurrence, and two patients have developed very high levels but have not yet developed other evidence of recurrent disease. It is concluded that raised CEA levels in lung cancer are infrequent, but in those patients who have or develop raised levels sequential investigation may be of value in monitoring response to treatment and clinical coourse.