Serum tissue polypeptide antigen (TPA) has been determined in 271 patients with pleural or pulmonary neoplastic diseases (131) or with benign lung diseases (140). Using a cut-off of 95 U/L, TPA has a sensitivity and specificity of 65%. Therefore, TPA serum determination can suggest a diagnosis of malignancy, but its evaluation, as a single test, is not useful to differentiate between malignant or benign disease. TPA can be useful as an ancillary test in the follow-up of lung cancer therapy.