The incidence rate of testicular cancer has been steadily increasing during the last 50 years, and only cryptorchidism, i.e. undescended testes, has been identified as an important risk factor. An interplay between changing environmental factors and genetic susceptibility e.g. in foreign compound metabolizing enzymes, may have important influences on the risk. The aim of this study was to investigate if glutathione S-transferase mu (GST mu) deficiency, which in previous studies has been associated with malignant melanoma and cancers of the lung and bladder, is a risk factor of testicular cancer. Three hundred and seventy-eight men participated (80 seminomas, 104 non-seminomas and 194 controls) in a population-based case-control study. The phenotype of GST mu was determined in 366 men by ELISA, the genotype was determined in 324 men by polymerase chain reaction. The concordance between geno- and phenotype was 94.4%. The odds ratio of having the GST mu negative phenotype and testicular cancer was 1.08, (0.72-1.64; 95% confidence interval (CI)), and the odds ratio of having the GSTM1 null genotype and testicular cancer was 1.10; CI95% (0.71-1.70). This study provides no evidence of an association between phenotypically determined GST mu deficiency or GSTM1 null genotype and testicular cancer. The narrow confidence intervals rule out GST mu as a major single risk factor for testicular cancer.