Sixty-one Wilms' tumors (WTs) from 59 patients who received preoperative therapy were studied. Twenty-seven WTs from 26 patients who did not receive preoperative treatment were also reviewed as controls. Marked and diffuse morphological changes occurred in treated cases. Necrosis affected mostly undifferentiated and replicating elements and was extensive, up to 90% of tumor mass. Minimal residual tumor, permitting recognition as Wilms', was always spared. Epithelial and rhabdomyoblastic components were more resistant to treatment; moreover, they appeared to be susceptible to differentiation and maturation. Necrosis and muscle cell differentiation seemed to have prognostic implications. Cases with extensive necrosis (greater than 90%) had a better outcome, although the difference was not statistically significant. The rhabdomyoblast/tumor mass ratio, after treatment, appears to carry prognostic meaning. Chemotherapy had no apparent effect on anaplasia.