1637 parathyroid gland dissections were performed on 589 unselected autopsy preparations in order to study both pathomorphology and functional aspects in an unselected group of patients. In no case had primary or tertiary hyperparathyroidism been suspected clinically; but in about 10 patients on chronic hemodialysis, a secondary hyperplasia of the glands was known. In about 33% of the cases pathological changes were observed: 3 chief cell adenomas, 3 adenomas combined with hyperplasia, 12 primary nodular hyperplasias including 6 microadenomas and 145 cases with secondary regulative hyperplasia. Cysts, purulent parathyroiditis, lipomatous pseudohyperplasia, metastases of carcinomas, hemorrhagic infarctions and amyloidosis were also detected in serial sections of the parathyroid glands. The remarkably large group of secondary hyperplasias (about 20%) was divided into 3 categories according to weight and cytology. The first category (118 cases) was characterized by a lack of the normal age-related involution of the parenchyma and prevalence of light (active) chief cells. Statistical evaluation of the other autopsy findings, histomorphology of the bone in particular, led to the conclusion that this group represented a slightly activated gland with possibly reversible hyperplasia, mostly connected with pathological alterations in the kidney and with minimal changes in bone morphology consistent with hyperparathyroidism. The microadenomas displayed different levels of endocrine activity as demonstrated by bone histology in accordance with the cytology of the adenomas (dark or resting chief cells versus active chief cells and waterclear cells). No specific alterations with respect to endocrine function could be attributed to diffuse and nodular hyperplasia of oxyphil (oncocytic) cells except their increase with age and arteriosclerosis, especially in women.