Although the commonest familial form of primary hyperparathyroidism is parathyroid hyperplasia, a few families have manifested parathyroid adenomas. We describe a family in which four members developed cystic parathyroid adenomas. Although calcium levels returned to normal after resection of the adenoma, a second adenoma would often develop several years later (we have termed this temporal sequence adenomatosis). Each adenoma had a cystic histologic appearance, and three of four normal-sized parathyroid glands also contained many cysts. No other endocrine tumors have appeared, but in three patients the hyperparathyroidism was complicated by fibrous maxillary or mandibular tumors that resembled ossifying fibromas rather than the brown tumors generally found in patients with hyperparathyroidism. Each patient with an adenoma was hypercalciuric, but two were obligate carriers of hypocalciuric hypercalcemia. This familial occurrence of the rare cystic parathyroid adenoma suggests the presence of a distinct hereditary syndrome. The genetic basis may be the simultaneous inheritance of familial hypocalciuric hypercalcemia and another trait that may increase the urinary excretion of calcium.