To evaluate the ability of gray-scale renal ultrasonography to distinguish cystic neoplasms from benign cysts, 182 cysts or cyst-like masses were reviewed retrospectively by three radiologists with varying degrees of experience in nephrosonography. All cases were proved by needle puncture or surgery. An unequivocal diagnosis of "cyst" based solely on ultrasonography was 98% accurate, with 2% being due to hematomas, localized hydronephrosis, or septa within the cyst. No cystic neoplasms were mistaken for benign cysts. These results indicate that experienced observers using proper technique will rarely be in error in distinguishing non-neoplastic cystic masses from cystic neoplasms by articulated-arm gray-scale ultrasonography, and that routine needle puncture may be unnecessary in such cases.