Some families seem to have an increased risk of several different cancers and a reduced risk of others. Either genetic predisposition or a shared environment may explain this familial clustering, and the type of cause can affect how family members should be advised. We used data from a case-control study to examine the risk of cancer in the mother, sisters and brothers of men with testicular cancer. Our results show a significant relative risk (RR=1.7; 95% confidence interval (CI): 1.05-2.6) of cancer for sisters of testicular cancer patients in comparison with the sisters of controls. When data were combined for brothers and sisters, the RR for all cancers was 1.53 (CI: 1.1-2.3). Despite the limitations of our data, there is evidence for cancer clustering in the families of testicular cancer patients. Unfortunately, the evidence is consistent with either a genetic or environmental etiology.