Intrathyroidal hyperplastic parathyroid glands were responsible for primary hyperparathyroidism (PHPT) in two of three members in a family. The third had an extrathyroidal parathyroid "adenoma". Both intrathyroidal parathyroid (IThP) hyperplastic glands were the largest ones removed at the time of surgical cure. A review of the literature confirmed our postulate of a higher incidence of familial cases among patients with hyperparathyroidism and IThP with an incidence of 10.34% of IThP in familial cases versus a 4.2% in non-familial cases with PHPT. This contrasts with an incidence of 0.1% of IThP in normal patients. We hypothesize that stimulation of IThP tissue by surrounding calcitonin-producing C-cells might play a role in the seemingly preferential IThP hyperplasia. Recognition of this syndrome of Familial IThP Hyperplasia is important in order to avoid unnecessarily aggressive surgery for hyperparathyroidism.