The synthesis of the active form of vitamin D, 1,25-dihydroxyvitamin D (1,25-(OH)2D), is thought to be relatively insensitive to the serum concentration of its precursor, 25-hydroxyvitamin D (25-OH-D). We compared the effect of oral administration of 25-OH-D3 (50 micrograms per day for one month) on serum concentrations of calcium, phosphate, parathyroid hormone, 25-OH-D, and 1,25-(OH)2D in five healthy adults and in six patients with primary hyperparathyroidism. In normal adults the mean (+/- S.D.) serum level of 25-OH-D rose from 18 +/- 9 to 136 +/- 47 ng per milliliter; no significant changes were observed in the other serum levels. In contrast, comparable increases in the levels of circulating 25-OH-D in patients with primary hyperparathyroidism caused a consistent slight rise in serum calcium and phosphate levels, a partial suppression of parathyroid hormone, and a sharp increase in the level of 1,25-(OH)2D. During this period a significant positive correlation was found between serum concentrations of 25-OH-D and 1,25-(OH)2D (P less than 0.001). These results provide evidence that in patients with primary hyperparathyroidism, levels of circulating 1,25-(OH)2D may be more dependent on the prevailing serum concentrations of 25-OH-D than they are in normal adults.