The cytologic features of 17 cases of Wilms' tumor are described. All three elements identifiable histologically are usually present in the fine needle aspiration smears. In our cases the blastemal component predominated, with varying degrees of epithelial and stromal differentiation, which was identifiable in the form of skeletal and smooth muscle. Early epithelial differentiation was identified as rosette-like and gland-like structures, groups of cells with peripheral palisading and spherical, tightly bound clusters of cells. More-differentiated epithelial cells were seen individually, in small, loose clusters, in sheets with a honeycomb appearance and as elongated tubules. Coarse, magenta, granular material was seen in the background of the smears in the majority of our cases. Fine needle aspiration under ultrasound guidance to accurately localize the tumor and a posterior approach to avoid peritoneal spillage are recommended. In difficult cases, a cell block of the aspirated material is a useful adjunct.