Aspiration biopsy of mass lesions of the kidney is a diagnostic procedure whose potential benefit to patients with equivocal radiologic findings, suspected metastases, palpable flank masses, nonfunctioning kidneys and even cortical cysts has not been fully explored. Over the past 7 years 163 renal aspiration specimens from 152 patients have been examined. Diagnostic yield was enhanced by use of modified "fine" needles with notched tips so that tissue fragments for histology as well as smears for cytology were obtained in 89% of solid tumors. The availability of aspirated tissue contributed significantly to pathologic classification and often spared patients additional surgery for confirmation of the cytologic diagnosis. Among the 152 cases, there were 35 with renal cell carcinomas, a sufficient number for detailed semiquantitative evaluation of their cytologic features. The remaining cases comprised metastatic carcinomas, lymphomas, transitional cell carcinomas, oncocytomas, cortical cysts, and miscellaneous conditions, e.g., abscesses, nonfunctioning kidneys, and hematomas. Overall, the aspiration biopsy determined the nature of the renal mass in 141 cases (93%). False-negative interpretations were due to insufficient diagnostic material in all but one instance. There was one false-positive result.