In order to evaluate the association between male subfertility and the subsequent risk of testicular cancer, a population-based case-control study was conducted in The Danish population. Cases were identified in the Danish Cancer Registry; controls were randomly selected from the Danish population using the computerized Danish Central Population Register. The men were interviewed by telephone; 514 cases and 720 controls participated. A reduced risk of testicular cancer was associated with paternity (relative risk = 0.63; 95% confidence interval: 0.47-0.85). In men who prior to the diagnosis of testicular cancer had a lower number of children than expected on the basis of their age, the relative risk was 1.98 (95% confidence interval: 1.43-2.75). There was no corresponding protective effect associated with a higher number of children than expected. The associations were similar for seminoma and non-seminoma, and were not influenced by adjustment for potential confounding factors. These data are consistent with the hypothesis that male subfertility and testicular cancer share important aetiological factors.