The cytologic findings observed in fine needle aspiration (FNA) biopsy smears from ten cases of Wilms' tumor are reported. Blastemal cells, small, round cells with slightly oval nucleoli and fine, evenly dispersed chromatin, were recognized in all cases. Stromal (90%) and epithelial components (40%) were also present. Additional recognizable features were anaplasia (in two of three cases having histologic anaplasia), rosettes (four cases), necrosis (six cases) and inflammatory cells (six cases). In six cases, small fragments of tissue were also recovered for histologic examination after paraffin embedding. There were no complications attributable to the procedure. FNA cytology may be used as the sole diagnostic procedure for Wilms' tumors when preoperative chemotherapy protocols are in use.