We retrospectively analysed the incidence and risk factors of treatment-related death in the treatment of chemotherapy- and thoracic radiotherapy-naïve patients with lung cancer. Between July 1992 and December 1997, 1799 patients were diagnosed as having lung cancer in our hospital and 926 patients received chemotherapy and/or thoracic radiotherapy. 25 patients (2.7%) died from toxicity of the treatment, 10 from pneumonia, 7 from radiation pneumonitis, 6 from sepsis, 1 from perforation of the small intestine and 1 for an unknown reason. 18 patients (2.3%) died from chemotherapy-related toxicity. The incidence of treatment-related death (TRD) from chemotherapy was highly correlated with the performance status (PS), PS 0: 0.7%, PS 1: 2.2%, PS 2: 4.0%, PS 3: 7.7% and PS 4: 25% (P=0.004). 7 patients (1.6%) died from pneumonitis after thoracic radiotherapy. Multivariate analyses demonstrated that poor PS (relative risk (RR): 1.95, 95% confidence interval (CI): 1.05-3.65, P=0.034) and chemotherapy using the cisplatin+vindesine+mitomycin C regimen (RR: 9.36, 95% CI: 1.29-68.0, P=0.027) are associated with treatment-related death from chemotherapy. Pulmonary fibrosis identified on a plain chest X-ray film (RR: 165.7, 95% CI: 8.79-3122, P<0.001), the combination of cisplatin+irinotecan (RR: 120.5, 95% CI: 2.90-4993, P=0.012), advanced age (RR: 1.17, 95% CI: 1.002-1.37, P=0.047), and elevated lactate dehydrogenase (LDH) (RR: 10.4, 95% CI: 1.20-90.2, P=0.033) were also associated with treatment-related death from thoracic radiotherapy. The administration of mitomycin C in addition to cisplatin-based regimens for patients with lung cancer should be carefully considered.