Abstract
The utility of ultrastructural evaluation of eosinophilic renal cell neoplasms is illustrated by two case studies. The differential diagnosis between granular renal cell carcinoma and renal cell oncocytoma may be difficult. Ultrastructural demonstration of the presence of abundant mitochondria is useful in the definitive diagnosis of fine-needle aspiration specimens, those neoplasms with nuclear pleomorphism, or in patients requiring renal parenchymal sparing surgery.