Cytogenetic analyses of 85 testicular germ cell tumors, of which 54 were karyotypically abnormal, showed recurrent breakpoints at chromosome bands 1p36, 1p13-1qh, 11q23, 19q13, and the pericentromeric regions of the acrocentric chromosomes. Postchemotherapy tumors had significantly more rearrangements of bands 3p25-p26, 6q16-q21, 8p22-p23 when compared with untreated tumors, while untreated tumors had more rearrangements of 9p22-p24 when compared with postchemotherapy tumors. Frequent breakpoints also were identified at 15q15 and 9qh in untreated tumors. Tumors of different histopathology, clinical stage, and treatment status showed no significant differences in the frequencies of i(12p)-positive and i(12p)-negative tumors.