Flow cytometric DNA analysis was carried out in 54 patients with testicular germ cell tumors (GCTs) experienced at our hospital, to evaluate the clinical relevance of DNA index (DI) and provide some insight into the pathogenesis of testicular GCTs. Histological types with their incidences were seminomas in 31 patients and nonseminomatous germ cell tumors (NSGCTs) in 23 adults. DNA ploidy and DI were analyzed by flow cytometry in 158 paraffin embedded samples; 2.9 samples per case on the average. This study revealed that 52 cases (96%) of evaluable 54 adult GCTs were DNA aneuploid, while DNA diploid tumors were observed in only each one case of NSGCT and seminoma. There was a significant difference (p less than 0.01) between the distribution of DIs in adult NSGCTs (median DI = 1.50) and that in pure seminomas (median DI = 1.85). Although we found no significant correlation between DI and clinical staging of Japanese Urological Association, on the basis of Indiana University staging system, the median DI in NSGCT patients of the advanced extent was lower than those of the other extents. DNA heterogeneity was observed only in 4 of 23 NSGCT patients (17%) and 3 of those 4 patients were assigned to advanced extent. These data suggest that the lower DI and the presence of DNA heterogeneity may have prognostic relevance for NSGCTs.