Although many risk factors have been proposed for the aetiology of testicular cancer, only a history of cryptorchidism is well established. All risk factors previously suggested have been explored in this study. This population-based case-control study was carried out in nine health regions in England and Wales and included 794 men, aged 15-49 years, diagnosed with a testicular germ cell tumour between 1 January 1984 and 30 September 1986, each with an individually age-matched control. Cases and controls were interviewed and data were abstracted from their general practitioner notes. Participation rates for cases and controls were 92.0% and 83.1% respectively. Where possible the mother of each interviewed man was sent a postal questionnaire for self-completion. Testicular trauma at least 2 years prior to diagnosis was associated with an odds ratio (OR) of 2.00 [95% confidence interval (CI) 1.54-2.61]. Ever having had a sexually transmitted disease was also associated with an increased risk (OR = 2.22, 95% CI 1.46-3.39). There was little evidence of an association with cigarette smoking. Sporting activity had a protective effect. Detailed exploration of testicular temperature (wearing of tight underpants, jeans or trousers, hot baths and central heating) failed to reveal any relationship with risk of testicular cancer. There were no clear occupational associations.