DNA ploidy in the primary tumor from patients with nonseminomatous testicular germ cell tumors clinical stage I. The DNA stemline ploidy was assessed in paraffin-embedded, formalin-fixed primary tumor tissue from 68 patients with nonseminomatous germ cell cancer (NSCGT) clinical Stage I (CS I). Forty-three patients had a single aneuploid (greater than 1C) DNA stemline, whereas 24 patients had multiple aneuploid stemlines. In one tumor there was no evidence of an abnormal DNA stemline. Most DNA stemlines had DNA indices around the 3c value. In 13 patients there was a good correlation between the DNA stemline values observed in the primary tumor and in the retroperitoneal lymph node metastases. No correlation was found between the DNA index and the histologic subclassification or the metastatic behavior. The size of the S-phase fraction did not appear to be predictive of subclinical metastases. In CS I patients with NSCGT determination of DNA stemline values does not yield information of predictive or prognostic significance but may contribute to the understanding of the pathogenesis of NSCGT. 