The study of multiple primary neoplasms may provide more insight into the pathogenesis of specific cancers and, secondly, it addresses the issue of treatment-related induction of second tumors. 584 consecutive patients with testicular germ cell tumors treated during 1969-1992 in Berlin were retrospectively analyzed for the prevalence of multiple primary neoplasms. 23 patients (16 pure seminoma, 7 nonseminoma) developing nontesticular malignancies in addition to testis cancer were identified (3.9%, 95 confidence intervals ± 1.6%). The mean age at the time of the testis cancer was 42.8 years, and mean age at the time of nontesticular malignancy was 50.2 years. In 4 cases, the nontesticular malignancy preceded testis cancer, in 3 patients both neoplasms occurred simultaneously, and in 16, nontesticular malignancies developed subsequently to testis cancer. In 4 patients, triple malignancies were observed. Bladder carcinoma and bronchogenic cancer with each 3 cases were the most frequent second neoplasms seen in these patients. 9 consecutive cancers developed within radiation fields, the median latency period in these cases was 12 years. None of the subsequent cancers developed after chemotherapy. It is concluded that genetic predisposition and radiation effects are the most important factors contributing to the occurrence of multiple primary neoplasms in patients with testicular germ cell cancer. The high rate of multiple primary neoplasms in these patients lends further evidence to the theory of genetic anticipation of germ cell cancer