We karyotyped two histologically distinct components with different metastatic behavior of a testicular nonseminomatous germ cell tumor. The two components showed an almost identical chromosomal pattern. These almost identical karyotypes of the two components with different metastatic potential suggest that the difference in biologic behavior might result from subtle differences (on microscopic or submicroscopic level) in chromosomal pattern or that these differences are predominantly epigenetically determined and depend primarily on the lineage of differentiation of the tumor component. Trophoblastic differentiation results in an aggressive, angioinvasive tumor but in development of teratoma in a tumor with low malignant potential.