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.

The cells and the tumor that they compose are often benign but sometimes may be premalignant or malignant.

Oncocytoma is essentially a subtype of null cell tumor and does not produce hormone and has the additional feature of oncocytic changes on light microscopy and substantial number of mitochondria in the cytoplasm under the electron microscope.[

As no hormone is produced by null cell adenomas and oncocytomas, they are not associated with clinical manifestations resulted from hormone over production.

However, one case of prolactin secreting oncocytoma presenting as galactorhea–amenorhea syndrome and another with Cushing's syndrome have been reported ^{1) 2)}