Besides asbestos exposure, the factors that determine susceptibility to malignant mesothelioma are unknown. We evaluated the risk of GSTM1 null genotype and slow acetylation-associated NAT2 genotype for malignant mesothelioma in relation to asbestos exposure. Both the GSTM1 null genotype and the NAT2 slow acetylator genotype placed individuals at about 2-fold increased risk of developing malignant mesothelioma [odds ratio (OR) = 1.8, 95% confidence interval (CI) = 1.0-3.5 and OR = 2.1, 95% CI = 1.1-4.1, for the GSTM1 and NAT2 genes, respectively]. When the patients were divided into low/moderate and high exposure groups according to their asbestos exposure histories, the effect of the at-risk genotypes was mostly attributable to the high exposure groups (OR = 2.3, 95% CI = 1.0-5.6 and OR = 3.7, 95% CI = 1.3-10.2, for the GSTM1 and NAT2 genes, respectively). The individuals with combined GSTM1 and NAT2 defects had about a 4-fold risk of developing malignant mesothelioma compared to those with the GSTM1 gene and NAT2 fast acetylator genotype (OR = 3.6; 95% CI = 1.3-9.6). Moreover, the risk among subjects highly exposed to asbestos with the double at-risk genotype was more than 7-fold greater compared to those with the more beneficial genotypes of both GSTM1 and NAT2 genes (OR = 7.4; 95% CI = 1.6-34.0).