The best treatment of patients with clinical Stage I nonseminomatous germ cell testicular tumors (NSGCTT) remains controversial. Archival formalin-fixed paraffin-embedded testicular tumor specimens from 23 patients diagnosed between 1977 and 1988 were available for analysis. All patients had clinical Stage I NSGCTT, and the specimens were prepared using Hedley's technique and propidium iodide staining prior to flow cytometric ploidy analysis. Tumors from 3 patients (13%) were classified as DNA diploid; the remaining 20 tumors had DNA aneuploid patterns. Eleven patients had retroperitoneal lymph node dissection (RPLND); 2 (18%) were upstaged to pathologic NSGCTT Stage IIA, and 1 had a subsequent relapse in the contralateral testicle. Among 12 patients in the surveillance protocols, 3 (25%) had tumor recurrence. All patients who were either upstaged to pathologic NSGCTT Stage IIA or had tumor recurrence while under surveillance had DNA aneuploid patterns. The 3 patients with DNA diploid tumor patterns had been in the surveillance protocols. To date, with fifteen months minimum follow-up, none has had tumor recurrence. Such data suggest that flow cytometric DNA ploidy analysis, when used as an adjuvant to more traditional prognostic parameters, may help to identify a group of patients with clinical Stage I NSGCTT at very low risk for recurrence and perhaps well-suited for surveillance management.