Serum concentrations of 1,25-dihydroxyvitamin D [1,25-(OH)2D] and immunoreactive parathyroid hormone were measured before and for 7 months after the removal of a 15-g parathyroid adenoma from a 44-yr-old woman with primary hyperparathyroidism and severe osteitis fibrosa cystica. Despite the fall in parathyroid hormone levels from preoperative levels of 20 to 1--2 ng/ml after surgery (normal, up to 1.2 ng/ml), serum 1,25-(OH)2D concentrations remained markedly elevated (156 pg/ml) preoperatively; 124 pg/ml 17 weeks postoperative), approaching the normal range (18--56 pg/ml) only after 5 months (65 pg/ml). Hypocalcemia and hypophosphatemia persisted despite oral 1,25-(OH)2D3 (1 and 2 micrograms/day) and large doses of (oral and iv) calcium gluconate (up to 30 g/day). Healing of the skeletal lesions, reversal of the myopathy, and return of 1,25-(OH)2D circulating levels to normal corresponded to the time when serum phosphate became normal. The stimulus for the persistently elevated serum 1,25-(OH)2D levels may have been hypocalcemia per se, low serum phosphate, or an unidentified signal that paralleled serum phosphate, as serum PTH levels remained in the upper normal range throughout the recovery period.