Two different types of amyloid deposits in pituitary adenomas are described herein. The deposits of the first type are stellate and perivascular masses, with fibrillary inclusions occurring in the intercellular space. No immunoreactive cytokeratin can be detected in the deposits, and the masses are secreted by the adenoma cells. A second, rarer type is characterized by spheroids (40 to 1500 micron in diameter) in which immunoreactive cytokeratin fibrils are present. The fibrils originate in the adenoma cells adjacent to the deposits. The amyloid nature of both types of deposits can be proved by intense red staining with Congo red, green dichroism, and positive immunostaining with antibodies raised against serum amyloid P component. Adenomas with amyloid spheroids are very rare, and most have been proved or suspected prolactin-producing adenomas.