An oncocytoma is a tumor made up of oncocytes, epithelial cells characterized by an excessive amount of mitochondria, resulting in an abundant acidophilic, granular cytoplasm.

[citation needed] Renal oncocytoma is thought to arise from the intercalated cells of collecting ducts of the kidney.

The histopathology is marked by sheets of large, swollen polyhedral epithelial oncocytes, which are granular acidophilic parotid cells with centrally located nuclei.

[citation needed] Thyroid oncocytomas (also known as Hürthle cell tumours) can be benign (adenomas) or malignant (carcinomas).

There are no reliable cytologic features which distinguish oncocytic adenomas from carcinomas and the only criterion for a diagnosis of malignancy is the identification of transcapsular or vascular invasion.