The current report describes a secondary malignancy developing in a retroperitoneal mature residual lesion 6 years after chemotherapeutic treatment of a disseminated nonseminomatous testicular tumor. The histologically malignant component was not present in the primary tumor and consisted of polygonal and fusiform cells with focal tubular formations, resembling primitive neuroectodermal tissue. Immunoperoxidase staining for alpha-fetoprotein and the beta-subunit of human chorionic gonadotropin remained negative, whereas focal positivity for S100 protein was observed. Neuron specific enolase positivity was equivocal. The DNA contents of both the mature components in the primary and the metastatic retroperitoneal tumor and in the various malignant components of the primary tumor, were in the hypotriploid range. In the malignant component of the retroperitoneal metastasis, a hypertriploid peak was observed. These findings suggest further clonal evolution in a phenotypically mature, genotypically abnormal residual metastatic tumor after chemotherapy. It is stressed that the mature appearance of the residual lesions may be deceiving and that these lesions are highly susceptible to resume malignant behavior.