Single-cell DNA measurements were performed on Feulgen-stained nuclei in 4 mu tumour sections from 30 patients with Wilms' tumour, all treated with initial nephrectomy. Fifteen patients died from their tumours within 2 years of nephrectomy. The remaining 15 had survived for at least 10 years without evidence of recurrence. Based on the histograms, the tumours were classified as being euploid (E type) or aneuploid (A type). Twenty tumours were euploid and 10 aneuploid. The euploid tumours predominated in the group of survivors. The group of 10 aneuploid tumours contained 7 tumour deaths. The tendency for aneuploid tumours to be associated with early tumour death, however, was not statistically significant.