Serum CA 125 levels were evaluated in 130 healthy subjects and 133 patients with untreated pulmonary lesions. These were 33 patients with benign pulmonary conditions and 100 with lung cancer. The mean concentration of CA 125 was higher in patients with lung cancer (37 +/- 81 U/ml) than in those with nonmalignant disease (4.2 +/- 5.7 U/ml) (P less than 0.01). In the healthy control group CA 125 concentrations were significantly lower (0.63 +/- 1.5 U/ml) (P less than 0.001). In patients with lung cancer the concentration of this tumor marker was related to the tumor-node-metastasis (TNM) stage. At a cut-off value of 15 U/ml, CA 125 had a sensitivity of 44%, specificity of 100%, positive predictive value of 100%, and negative predictive value of 65% with respect to healthy subjects; in patients with benign pulmonary conditions, these values were 44%, 94%, 94%, and 31%, respectively. At this cut-off value, a correlation between the respectability prognosis and the likelihood of survival 24 months posttreatment was observed. These findings suggest that CA 125 can be used as an adjunctive test in the management of patients with lung cancer patients.