Testicular germ cell tumors of adolescents and adults, both seminomas (SE) and nonseminomas (NS), are aneuploid, and classical karyotyping demonstrated a specific pattern of gains and losses. More recently, these data have been supported by in situ hybridization and comparative genomic hybridization (CGH) on a limited number of samples. Interpretation of CGH results is complicated by the intermediate ploidy of these tumors (3-4 n for SE and 2-3 n for NS). To circumvent this problem, this particular study was undertaken. CGH was performed on 8 SE and 10 NS, after which two single chromosome normalizations were applied, one for chromosome 4 (found to be associated to the lower ploidy level of the tumor) and one for chromosome 8 (found to be associated with the higher ploidy level of the tumor) Using this modified CGH interpretation method, chromosomal regions with a similar copy number of chromosome 4 and 8 were identified as belonging to the lower and higher ploidy level, and the regions below chromosome 4 and above chromosome 8 were identified as lost or gained outside the ploidy range of the tumor, respectively. Our results are in accordance with earlier findings, however they add novel data, including comparison of SE and NS. This approach reveals relevant information about the chromosomal constitution of testicular germ cell tumors, leading to a better understanding of the pathogenesis of these tumors.