The association between lung cancer and occupations with probable exposure to diesel exhaust has been examined in several studies in the past with inconclusive results. We report a case-control study among 2,584 cases and 5,099 hospital controls. The crude odds ratio for probable exposure was 1.31 (95% Cl 1.09, 1.57), but adjustment for smoking and other confounders reduced the estimate to 0.95 (95% Cl 0.78, 1.16). Similar results were observed for truck drivers, the only occupational category large enough for separate analysis. Data on self-reported exposure for a subset of 477 cases and 946 controls revealed a crude odds ratio of 1.45 (95% Cl 0.93, 2.27) which was reduced to 1.21 (95% Cl 0.78, 2.02) after controlling for smoking and other confounders. A duration-response relationship was suggested (P less than 0.12) only for self-reported exposure. Cigarette smoking was the predominant confounder in the analysis. Problems arising from biases and confounding in the present and in other previously published reports on the association between diesel exhaust exposure and lung cancer risk are discussed, and a review on the evidence of human carcinogenicity of diesel exhaust exposure is presented.