DNA content (ploidy status and proliferative fraction) was retrospectively studied by flow cytometry on archival material from 44 Wilms' tumors of the kidney to evaluate intratumoral DNA heterogeneity and correlate the findings with clinicopathologic factors and patients' outcome. Analyses of 168 blocks from these neoplasms (average 4 per case) revealed diploid DNA in 24 (54.5%) and aneuploid DNA in 20 (45.5%). Thirteen (29.5%) Wilms' tumors demonstrated intratumoral DNA heterogeneity. No statistical correlation between DNA content and age, gender, rumor size, tumor stage, histological subtypes, and clinical outcome was found. Significant association, however, was found between DNA ploidy heterogeneity and unfavorable histology, and death due to disease. Our results indicate that approximately one third of Wilms' tumors manifest intratumoral heterogeneity and this accounts, at least in part, for the variable incidence of DNA ploidy reported and may be associated with pernicious biological feature.