To investigate the role of vitamin D metabolites in the pathogenesis of pseudohypoparathyroidism, we studied an elderly man with a unique variant of the disease, which was characterized by hypocalcemia, elevated serum parathyroid hormone (513 +/- 13 pg per milliliter, mean +/- S.E.M., normal, less than 450) but normal renal responses (phosphate and cyclic AMP) to exogenous parathyroid extract. Treatment with parathyroid extract did not produce a calcemic effect, suggesting an isolated skeletal hyporesponsiveness to parathyroid hormone. Although 25-hydroxyvitamin D levels were not reduced, levels of 1,25-dihydroxycholecalciferol were extremely low (0.52 ng per deciliter; normal 3.3 +/- 0.06, S.D.). Treatment with 1,25-dihydroxycholecalciferol (1 microgram by mouth per day for four days) increased circulating levels to normal (4.60 ng per deciliter) and restored to normal the calcemic response to parathyroid (change in calcium 3.0 mg per deciliter). These data suggest that 1,25-dihydroxycholecalciferol deficiency may explain the skeletal resistance, but not the renal resistance, often present in classic pseudohypoparathyroidism.