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We evaluated the relation between occupational exposure to diesel exhaust and cancer of the lung in a meta-analysis of 29 published cohort and case-control studies. Twenty-one of the 23 studies meeting the inclusion criteria had observed relative risk estimates greater than one. Pooled effect measures weighted by study precision indicated an increased relative risk (RR) for lung cancer from occupational exposure to diesel exhaust [RR = 1.33; 95% confidence interval (CI) = 1.24–1.44]. Subanalysis of case-control (RR = 1.33; 95% CI = 1.18–1.51) vs cohort studies (RR = 1.33; 95% CI = 1.21–1.47) and of studies that controlled for smoking (RR = 1.35; 95% CI = 1.20–1.52) vs those that did not (RR = 1.33; 95% CI = 1.20–1.47) produced results that did not differ from those of the overall analysis. On the other hand, cohort studies using internal comparisons (RR = 1.43; 95% CI = 1.29–1.58) showed higher relative risks than those using external comparisons (RR = 1.22; 95% CI = 1.04–1.44). Heterogeneity between studies was reduced when we stratified studies by the occupational setting in which exposure occurred. A positive duration-response relation was evident in those studies that were stratified by employment duration. This meta-analysis supports a causal association between increased risks for lung cancer and exposure to diesel exhaust.