The association between the wearing of tight underpants and trousers and risk of dyspermia has been analysed using data from a case--control study. Cases included infertile men with a diagnosis of unexplained dyspermia. Normospermic men of infertile couples were eligible as controls. In comparison with men usually wearing loose underpants, the odds ratio (OR) of dyspermia was 1.9 (95% confidence interval, CI, 0.9-4.1) in those wearing tight underpants. Likewise, the OR of dyspermia was 1.6 (95% CI 0.9-3.0) in men reporting usually wearing tight trousers (including jeans), in comparison with those wearing loose trousers.