Twenty-three bilateral testicular germ cell tumors (four synchronous and 19 sequential tumors) were investigated for potential risk factors. The incidence of maldescensus testis was not found to be higher than in patients with unilateral disease. The histologic findings of the first tumor did not have any effect on the incidence of the second tumor. In 21 patients (four synchronous and 17 sequential tumors), histocompatibility antigens (HLA) were determined; HLA-B14 was increased significantly in the sequential tumor group. Tendencies toward an increase of HLA-DR5 and HLA-DR7 also were found. The HLA-DR1, HLA-DR3, and HLA-DR4 showed a tendency toward a decreased frequency. Therefore genetic factors might be important in the development of sequential bilateral testicular cancers.