Thirtynine cases of primary parathyroid chief cell hyperplasia were separated into three categories on the basis of gross and microscopic morphology. The “classic” group included patients with obvious diffuse enlargement of all parathyroid glands and corresponding microscopic hyperplasia. In the “pseudoadenoma” group large nodular parathyroids were accompanied by other small glands. The latter, however, were hyperplastic microscopically, indicating diffuse abnormality. The third, “occult,” group was composed of patients whose parathyroid glands were no more than minimally enlarged yet showed subtle microscopic evidence of hyperplasia. Whereas the “classic” and “pseudoadenoma” groups were clearly related, they should be differentiated since the latter can masquerade as parathyroid adenoma. In such instances only the large adenomatous gland may be removed, leaving three smaller but abnormal glands behind. Despite the relatively small quantity of hyperplastic parenchyma in the “occult” group, no correlation was found between any type of hyperplasia and the clinical or laboratory features of these cases. Chief cell hyperplasia is probably a more common form of primary hyperparathy roidism than is generally recognized.