Functioning oncocytic adenoma of the pituitary.

A case of a functioning invasive pituitary oncocytoma in a 50-year-old man is reported. The serum prolactin was increased and immunocytochemical staining was positive for prolactin. In addition to numerous mitochondria within the neoplastic cells, structurally abnormal mitochondria with crystalline inclusions were also present. Another interesting feature was the presence of about 10 to 15% of non-oncocytic cells which showed prominent RER and Golgi apparatus as found in functioning pituitary adenomas. It is likely that these secretorily active cells were responsible for the functional activity of the tumour.