Familial occurrence of testicular cancer suggests a genetic predisposition to the disease. A genetic susceptibility may also be reflected by the occurrence of bilateral testicular neoplasms and the high rates of urogenital developmental anomalies in families prone to testicular cancer. In this study, the proportion of familial testicular cancer cases was analyzed retrospectively in a single-centre population of 693 testicular cancer patients treated between 1977 and 1997 and the relative risk (RR) for first-degree relatives of patients was estimated. In addition, the existence of bilateral testicular neoplasms and urogenital developmental anomalies in familial testicular cancer patients was evaluated. 24 of the 693 patients (3.5%) had a first-degree relative with testicular cancer. These 24 cases belonged to 17 families; in 7 of these 17 families both affected first-degree family members were part of the study population of 693 patients. Consequently, the 693 studied patients belonged to a total of 686 families. Thus, the actual proportion of familial testicular cancer was 2.5% (17 of 686 families). The familial cases consisted of 11 brother pairs, including 2 pairs of identical twins and 1 pair which also had two affected cousins, and 6 father-son pairs (in total 36 cases, 12 treated elsewhere). Estimates of the RR to first-degree relatives showed a 9- to 13-fold increased RR to brothers (P < 0.001) and a 2-fold increased RR to fathers (P = non-significant (n.s)) of testicular cancer patients. Among the 36 patients with familial testicular cancer, 2 (5.6%) had bilateral testicular cancer, 4 (11.1%) had undescended testis, 3 (8.3%) had inguinal hernia, and 1 (2.8%) showed renal hypoplasia. The present data on familial occurrence of testicular cancer may lend support to a role of genetic factors in the aetiology of testicular cancer.