In a case-control study of testicular cancer, 6 of 269 cases (2.2%) reported a first-degree relative with testicular cancer, compared to 1 of 259 controls (0.4%). Fathers and brothers of testicular cancer cases had a six-fold elevated risk of developing a testicular malignancy compared to men in the general population. Cryptorchidism was reported in a first-degree relative in 1 (17%) of the familial cases versus 7 of 259 (2.7%) controls and 14 of 263 (5.3%) cases with a negative family history for testicular cancer. One half of the 6 familial cases reported a first-degree relative with a groin hernia (all surgically repaired before age 12), compared to 12.7% of 259 controls and 10.3% of 263 nonfamilial cases. Three familial clusters identified through the case-control study were selected for clinical evaluation. One of the 6 surviving males with testicular cancer in these 3 families had undergone orchiopexy and inguinal herniorrhaphy at age 6 years, and one had a hydrocele associated with his testicular tumor. Of the 12 living fathers and brothers of these 6 men, 3 reported childhood inguinal hernias, two with coexisting hydroceles. One additional hernia and two additional hydroceles were detected during urologic evaluation of these healthy relatives. The high prevalence of cryptorchidism, inguinal hernias, and hydroceles among men in these families suggests that an underlying alteration in urogenital embryogenesis may be associated with the familial predisposition to testicular neoplasia.