Eight primary testicular germ cell tumors, one teratocarcinoma cell line, and one Leydig cell tumor were studied to determine the importance of modifications of the nucleolar organizer regions (NORs) in human testicular tumors. Cytogenetic analysis after silver staining showed active ectopic NORs in two primary embryonal carcinomas (EC) in the cell line and in single cells of each of two seminomas (S). In one EC, an ectopic NOR was localized to chromosomal region 1q4; the others were on unidentified rearranged chromosomes. All tumors in which ectopic NORs were observed were hyperdiploid and possessed marker chromosomes typical of human germ cell tumors. Quantitative DNA analysis was performed on three tumors: a teratocarcinoma (TC) and the Leydig cell tumor, which had provided no analyzable mitoses, and a seminoma which was cytogenetically diploid. In all three cases, the major populations were hyperdiploid. The results, in combination with those of an earlier study, provide evidence that active ectopic NORs are common in human testicular tumors.