An investigation of the role of fine needle aspiration in the assessment of renal masses was carried out on 132 consecutive patients, 11 of whom were children. Selection was based on the presence of a solid or mixed solid and cystic renal mass that could not be defined by radiology, either in symptomatic patients or in patients to be submitted to embolization of the renal artery and at high surgical risk. Histologic and clinical data showed 49% of the cases to have a malignant, predominantly (45%) primary disease of the kidney and the remainder to have a nonneoplastic lesion. In five cases, the primary lesion was in the adrenal gland (three neuroblastomas and two pheochromocytomas). Sensitivity, specificity and predictive values for positive results were, respectively, 0.93, 0.96 and 0.935 because of a false-positive diagnosis in a case of multilocular cystic nephroma. Furthermore, in 43 of 65 cases (66%), consisting of 33 renal cell carcinomas, 1 transitional cell carcinoma, 3 Wilm's tumors, 1 neuroblastoma, 2 pheochromocytomas and 3 metastatic lesions, the histologic type could be ascertained on the tissue yielded by the fine needle aspiration. The findings stress the usefulness of this method for the clarification of radiologically not unequivocal space-occupying lesions of the renal area, especially when, in addition to the smears, histologic sections of paraffin-embedded tissues are available. Ultrastructural and immunohistochemical studies are cost-effective mainly in pediatric patients.