Proliferation rates may determine prognosis under chemo- and radiotherapy independently. If so, distribution over lesions with and without aneuploidy should relate to tumor prognosis as defined by other parameters. Our aim was to study the distribution of (high) proliferation rates over WT related to ploidy status. Routine prospective image cytometry for ploidy and proliferation fraction, using 2.5 and 5c criteria based on published methods, was carried out on 14 nonselected cases of Wilms tumor. Aneuploidy was confirmed by karyotype. In one case cytological material was nonrepresentative. Out of 13 cases, 4 lesions were diploid (DNA index < 1.1; proliferation rates at 2.5c: 5.7, 13.5, 8.3, 8.6). Two cases were near diploid (DNA index: 1.1-1.25; proliferation rates at 2.5c: 17.7, 7.7). Aneuploidy (DNA index > 1.25) was found in seven cases (proliferation rates at 2.5c: 95.4, 95.5, 55.6, 33.0, 15.3, 30.0, 28.2). High proliferation rates in WT may be limited to aneuploid lesions. If good prognosis in WT is associated with aneuploidy, then the relationship between high proliferation rates and treatment sensitivity in WT warrants further study. Copyright © 1997 Taylor & Francis.