occurrence of lung cancer. The divergent results of these recent analyses do not negate the positive evidence this study provides for the weight-of-evidence evaluation. Three aggregate analyses of studies concerned with the relationship of diesel exhaust exposure and lung cancer risk are Cohen and Higgins (1995), Bhatia et al. (1998), and Lipsett and Campleman (1999). The Cohen and Higgins (1995) report is a qualitative review of 35 epidemiologic studies (16 cohort and 19 case-control) of occupational exposure to diesel engine exhaust published between 1957 and 1993. Control for smoking was identified in 15 studies. Six of the studies (17%) reported RR estimates less than 1, whereas 29 (83%) reported at least one RR greater than one, indicating a positive association. Twelve studies indicating a RR greater than 1 had 95% confidence intervals that excluded unity. The evidence suggests that occupational exposure to diesel exhaust from diverse sources increases the rate of lung cancer by 20% to 40% in exposed workers generally, and to a greater extent among workers with prolonged exposure. They also found that the results are not explicable by confounding due to cigarette smoking or other known sources of bias. Bhatia et al. (1998) found a small but consistent increase in the risk for lung cancer among workers with exposure to DE, in a quantitative meta-analysis of 23 studies that met criteria for inclusion. The observed RR estimates were greater than 1 in 21 of these studies. The pooled RR weighted by study precision was 1.33 (95% CI=