Glutathione S-transferase M1 (GSTM1) is a foreign compound-metabolizing enzyme with a heritable complete lack of activity in about 50% of Caucasians. GSTM1 deficiency may predispose individuals to urinary bladder cancer. Thus, a hospital-based case-control study was performed with 296 patients with bladder cancer and 400 controls, investigating this GSTM1 deficiency in relation to environmental risk factors and types of bladder cancer. Frequencies of the GSTM1 gene deletion (genotype, GSTM1*0/0) and of the allele variants A (mu) and B (psi) of the GSTM1-active trait were determined using an internal standard-controlled polymerase chain reaction technique. Moreover, in all patients GSTM1 expression was quantified in blood by an immunoassay. Of the cases, 59.1% had the GSTM1*0/0 genotype, in contrast to 50.7% of the controls (odds ratio, 1.40; 95% confidence limits, 1.02-1.92; P = 0.017). The odds ratio after adjustment for age and gender by logistic regression analysis was 1.54 (95% confidence limits, 1.12-2.13). Occupational risk was defined as previous employment in occupations with known increased bladder cancer risk, but the impact of GSTM1*0/0 was not significantly different in individuals with risk jobs versus those without. The greater proportion of the GSTM1-deficient individuals in the group with cancer was due to a lower frequency of carriers of GSTM1A. The odds ratio for the subgroup of individuals with the GSTM1B phenotype versus carriers of the GSTM1A phenotype in cases versus controls was 1.65 (95% confidence limits, 0.976-2.78; two-tailed Fisher's exact P = 0.057). Analysis of functional GSTM1 activity in a subset of 370 blood samples with the model substrate trans-stilbene oxide confirmed the genetic results and showed that 9 of 10 individuals with mu/psi heterodimers (genotype, GSTM1*A/B) had activities above the median of all genetically GSTM1-active individuals (24 pmol/min/1 x 10(6) lymphocytes; P < 0.01), indicating a gene dose relationship for GSTM1. GSTM1 expression in the urinary bladder endothelium detected by immunoassay and immunohistology corresponded to the genotype of the patients. It may be concluded from this study that the heritable GSTM1 deficiency is responsible for 17% (etiological fraction; 95% confidence limits, 2-30%) of bladder cancer cases.