The risk of a new primary cancer (NPC) among 77548 Finnish lung cancer patients from 1953 to 1995 was analysed by the histological type of the lung cancer. The relative risks were expressed as standardised incidence ratios (SIR, ratio of the observed and expected numbers of cases). During the follow-up, 1148 NPCs were observed among men and 152 among women. After exclusion of lung cancers, the risk of NPC was elevated in both males (SIR 1.07; 95% confidence interval (CI) 1.00-1.14) and females (SIR 1.21; 95% CI 1.02-1.42). The excess was larger among lung cancer patients with small-cell carcinoma and adenocarcinoma than those with squamous-cell carcinoma. In all major histological groups of lung cancer, significant excess risks were found for cancers of the larynx (SIRs 2.94-4.25), and bladder (SIRs 2.16-2.86). Significantly elevated SIRs were also found for cancers of the stomach (SIR 1.42; 95% CI 1.12-1.76) and kidney (SIR 2.18; 95% CI 1.56-2.97) in squamous-cell carcinoma; for brain tumours (SIR 3.26; 95% CI 1.20-7.09) in small-cell carcinoma; and for cancers of the prostate (SIR 1.68; 95% CI 1.21-2.27) and thyroid (SIR 3.79; 95% CI 1.23-8.85), and brain tumours (SIR 2.34; 95% CI 1.07-4.43) in adenocarcinoma. The risk of contracting NPC at sites where the majority of tumours are adenocarcinomas was elevated among patients with adenocarcinoma of the lung, but not among squamous-cell or small-cell carcinoma patients. In adenocarcinoma, the excess risks of several smoking-related cancers tended to be somewhat lower than those in the other two histological categories. The relative risk of a NPC among patients diagnosed with lung cancer in 1985-1995 was higher than that of patients from earlier periods in all comparable follow-up categories (up to 10 years), possibly suggesting that the increased use of cytostatic drugs had increased the risk of NPC.