Serum concentrations of neuron-specific enolase (NSE), carcinoembryonic antigen (CEA) and CA-50 antigen were determined in 168 consecutive patients with lung cancer. All three markers were significantly elevated compared with levels in 102 patients with non-malignant chest diseases. NSE and CEA varied significantly across histological lung cancer types, with most highly elevated serum levels in small cell lung cancer and adenocarcinomas, respectively. The overall diagnostic accuracy was 0.66 for NSE, 0.74 for CEA, and 0.62 for CA-50, implying that CEA best discriminated between lung cancer and benign chest diseases, while CA-50 was less efficient as a diagnostic marker. In multivariate analysis of the three markers combined, a positive predictive value of 95% for lung cancer could be achieved with a diagnostic sensitivity of 57%, with a cut-off level defined as 0.037.NSE + 0.052.CEA + 0.011.CA-50 > 1. In 22% of the cancer patients, the time from admission to histological or cytological lung cancer diagnosis exceeded 1 month. In 52% of these patients, the initial weighted tumour marker index was > 1, strongly implying the cancer diagnosis. The study lends support to the potential use of combined analysis of NSE, CEA and CA-50 as a complementary tool in the diagnosis of lung cancer.