We investigated infertility risk factors by conducting a population-based case-control study in the military population of the French town of Brest. Sixty couples who had sought medical advice for infertility of more than 12 months duration (cases) were compared with 165 couples who had had a child (controls). All the men in these couples had been employed by the military. The infertility risk factors studied were male and female medical factors, occupational and environmental exposures. We obtained age-adjusted odds ratios of 7.4 [95% confidence interval (CI): 1.4--39.5] for testis surgery, and 13.0 for varicocele (95% CI: 1.4--120.3) in men. In logistic regression, the age-adjusted odds ratio for men who had worked in a nuclear submarine was found to be 2.0 (95% CI: 1.0--3.7), and that for heat exposure was 4.5 (95% CI: 1.9--10.6). One limitation of this study is the lack of exposure measurements, especially for potential exposure to nuclear radiation (type of reactor used in nuclear-powered submarines, inability to obtain personal dosimeters worn by military personnel working in nuclear submarines). In conclusion, this study suggests that in this military population, having worked as a submariner in a nuclear-powered submarine, and having worked in very hot conditions, should be considered as risk factors for infertility.