Flow cytometry was used to determine the DNA ploidy pattern of 148 testicular germ cell neoplasms (seminomas and nonseminomas in pure and mixed histologic phenotypes) and in situ carcinoma (CIS) adjacent to these tumors. The great majority (96.0%) manifested aneuploid DNA contents with minimal intratumoral heterogeneity (2.5%). The mean DNA indices (DI) of CIS (1.7 +/- 0.18), pure seminoma (1.82 +/- 0.55), and the seminoma component of mixed germ cell neoplasms (1.76 +/- 0.13) were statistically similar. The mean DI of nonseminomas pure (1.46 +/- 0.29) or as a component of mixed tumors (1.43 +/- 0.32) was significantly lower (p greater than 0.001) than those of CIS and seminomas. Our data suggest that the similarity between the DNA indices of CIS and seminomas provide evidence that both lesions constitute a temporal evolutionary step in the progression of germ cell tumors and that nonseminomas may subsequently arise from either CIS or seminoma by further loss of chromosomal DNA. These characteristic findings support the nonstochastic theory for germ cell evolution and progression and may be useful in the clinicopathologic evaluation of testicular masses.