We analyzed, by light microscopy, specimens obtained at parathyroid surgery from 18 members of eight kindreds with familial hypocalciuric hypercalcemia. For comparison, similar analyses were performed with normal parathyroid glands (surgical or postmortem specimens) and with glands from patients with typical primary parathyroid hyperplasia. The average parathyroid parenchymal area in familial hypocalciuric hypercalcemia was 300 per cent of that in normal subjects (0.049 versus 0.15 sq. cm., p less than 0.0005) but significantly less than that in typical primary parathyroid hyperplasia. Thirteen of the 18 subjects with familial hypocalciuric hypercalcemia had one or more enlarged glands. Enlarged parenchymal areas were noted at all ages in familial hypocalciuric hypercalcemia, but there was a spectrum of histologic findings among the glands in each case, within most families, and between families. Mild parathyroid hyperplasia was a feature in most patients with familial hypocalciuric hypercalcemia who had undergone neck surgery. Simple quantitative analyses were useful in identifying parathyroid hyperplasia in the group with familial hypocalciuric hypercalcemia.