Testicular germ cell tumors (TGCT) of adolescents and adults are, for clinical and pathological reasons, divided in seminomas (SE) and nonseminomatous germ cell tumors (NS). Whether and to what degree these two entities are pathogenetically related is still controversial and a matter of debate. TGCT may contain both SE and NS components. Cytogenetic studies of the SE and NS components of these mixed TGCT might shed light on the pathogenetic relationship between both components. Separate cytogenetic analysis was performed on both components of three cases of mixed TGCT with both SE and NS components. The karyotypes of both components were compared with each other. In one case, the SE and NS component share eight different structural chromosomal abnormalities, indicating that the SE and NS component are pathogenetically closely related and have a common neoplastic pathway for a considerable length. Both components of the other two cases have, respectively, i(12p) and no structural chromosomal abnormalities in common. Our results, together with data from the literature, indicate that in mixed TGCT with SE and NS components, both components may have either a monoclonal or a polyclonal origin.