In a series of 381 consecutive patients with lung tumors and benign pulmonary diseases, we examined whether tumor markers CYFRA 21-1 (EIA, Boehringer, Mannheim), TPA-M (IRMA AB Sangtec Medical, Bromma, Sweden), TPS (IRMA, Beki Diagnostics AB, Bromma, Sweden), CEA and NSE (EIA, Roche, Basel) have the potential to contribute to clinical decision-making processes with respect to diagnosis and assessment of response to therapy. The sensitivity values of the marker tests in NSCLC (CYFRA 21-1: 44.4% > 3.9 ng/ml, TPA-M: 39.4% > 200 U/ml, TPS: 13.2% > 230 U/ml, CEA: 37.5% > 8.6 ng/ml), in SCLC (NSE: 61.9% > 14.0 ng/ml) and in pleural mesothelioma (CYFRA 21-1 and TPS: 36.4%) were found to be clearly inferior to the yield of standard cytopathological examinations (85-98%) when using the 95% specificity versus the group with benign pulmonary disease as cut-off values. Therefore, currently available tumor markers are of minor value in the primary diagnosis of lung tumors. After curative surgery (Ro) of NSCLC only CYFRA 21-1 levels dropped to the normal range within one week. The other markers simulated residual tumor mass by displaying elevated marker levels after surgery. During the monitoring of response to chemo-/radiotherapy the changes in marker levels were compared to the clinical assessment according to standard criteria of the WHO. The criteria defined for marker response were a 65% decrease for a partial response and a 40% increase of the marker levels for progressive disease. Concordant results were obtained in 59.4% of the cases for CYFRA 21-1 (TPA-M: 63.3%, TPS: 65.5%, CEA: 54.8%, NSE: 68.9%). Most discordant results were obtained in tumor remission due to an insufficient decrease in the markers. Progressive disease was most effectively indicated by CYFRA 21-1 in NSCLC 60%) and by NSE in SCLC (70.0%). It is concluded that increasing marker levels may contribute to clinical decision making, at least in helping to decide which patients should no longer treated by ineffective and toxic drugs.