The serum levels of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] were increased in 5 patients with primary hyperparathyroidism [60 .+-. 13 (SD) pg/ml; normal value, 33 .+-. 15 (SD) pg/ml] but fell rapidly after parathyroidectomy to values of 23 .+-. 9 (SD) pg/ml. This was accompanied by parallel decreases in the serum concentrations of Ca and immunoreactive parathyroid hormone. Over the following 5-35 days, the serum 1,25(OH)2D3 concentrations increased markedly to levels of 59 .+-. 17 (SD) pg/ml, which could most likely be explained by a stimulatory effect of the hypocalcemia per se on the renal production of 1,25(OH)2D3.