Serum carcinoembryonic antigen (CEA) levels were obtained before operation in 214 patients undergoing diagnostic tests for suspected bronchial carcinoma, and the results correlated with the postoperative, pathological stage of disease. Positive CEA levels (greater than 10 ng/ml) were observed in 40% (8/20) of stage 1, 58.5% (31/53) of stage 2, 85.2% (69/81) of stage 3, and 92.3% (24/26) of stage 4 patients with bronchial carcinoma. Furthermore, the mean CEA levels increased with stage of disease, and the differences between mean levels were found to be significant in stages 1 and 2 versus 3 and 4 (p less than 0.001). This suggests a positive correlation between the preoperative CEA level and tumour burden defined by pathological staging. When the results were compared with the histological type of lung carcinoma, CEA elevations occurred most frequently with adenocarcinoma, followed by undifferentiated and squamous cell carcinoma, reflecting perhaps the origin of this oncofetal antigen from the endodermally derived bronchial mucosa. These data indicate that preoperative serum CEA levels quantitatively reflect the extent of tumour assessed pathologically at operation and confirm the potential usefulness of this antigen as a biological tumour marker in the management of bronchial neoplasms.