A total of 33 specimens from 29 patients with residual stable teratoma and mature growing teratoma after chemotherapy was analyzed for clinicopathological and deoxyribonucleic acid (DNA) flow cytometric variables, as well as the presence of proliferative antigen and tumor marker levels in an attempt to explain their clinical behavior. We compared the flow cytometric and histological data of these stable and growing teratomas, and of nonseminomatous germ cell tumors before chemotherapy. The DNA content of residual teratoma did not differ significantly from that of the primary testicular tumors (1.38 versus 1.48). Histological analysis of stable and growing teratomas revealed no significant difference but proliferative cellular nuclear antigen was expressed mainly in the epithelial component of the growing teratoma. alpha-Fetoprotein, beta-human chorionic gonadotropin and carcinoembryonic antigen were elevated in the fluid of 6 excised teratomas, while concomitant serum levels were normal. The aneuploidy and elevated cystic fluid tumor markers confirm the malignant phenotype of post-chemotherapy residual teratomas. Therefore, complete surgical removal is essential despite the benign histological appearance.