Eighty-two per cent of tumour sections from 105 patients with lung cancer showed positive immunocytochemical localization of an anti-carcinoembryonic antigen (CEA) immunoglobulin free of antibody to normal cross-reacting antigen (NCA). The highest incidence was found in adenocarcinomas, and no association between staining and disease stage was found. There was a relationship between positive-staining tumours and preoperative and postoperative serum CEA levels of greater than or equal to 20 ng/ml, but the high incidence of CEA+, less than 20 ng/ml serum patients indicated that immunocytochemical localization was of little value in selecting patients for sequential serum monitoring. Staining for CEA was not prognostic but a preoperative serum CEA levels greater than or equal to 20 ng/ml was associated with a poor prognosis in patients undergoing radical surgery for lung cancer (P = 0.043). this prognostic effect of CEA was seen mainly in patients whose tumours showed the greatest immunocytochemical localization (P = 0.017) and in Stage III patients (P = 0.04).