Carcinoembryonic antigen (CEA) was measured in 147 patients at diagnosis of small cell lung cancer; 17% of patients with limited disease and 51% with extensive disease had an abnormal CEA level (greater than 10 ng/ml). The median level was higher in extensive than in limited disease (11 ng/ml and 5.8 ng/ml, respectively; P less than 0.001). Multivariate analysis showed CEA level greater than or equal to 50 ng/ml to be an adverse prognostic factor (P = 0.02); median survival at this level was shorter than at less than 50 ng/ml (7 and 46 weeks, respectively; P = 0.002). No consistent directional changes of follow-up CEA values were observed in patients with initially normal CEA levels, but normalization of levels occurred in complete responders. We recommend that CEA be measured in this disease at diagnosis as an additional prognostic factor and that patients with abnormal initial CEA levels have follow-up measurements to aid in evaluating response.