A 1.6-yr-old Hispanic boy with sparse hair, muscle weakness, severe growth retardation, and rickets was found to have hypocalcemia, secondary hyperparathyroidism, and high circulating 1,25-dihydroxyvitamin D [1,25-(OH)2D] levels. One year of treatment with a high dose of 1,25-(OH)2D3 (20 μg, orally, daily) resulted in marked subjective and radiological improvement; there was a parallel improvement in serum calcium, phosphorus, PTH, alkaline phosphatase, and osteocalcin concentrations. Soluble extracts from cultured skin fibroblasts did not bind [3H]1,25-(OH)2D3 using standard methods. To evaluate the possibility of receptors with abnormally low affinity, we tested for binding with [3H]1,25-(OH)2D3 concentrations higher than those usually used. In one experiment, there was a suggestion of low affinity binding. Hormone receptors with abnormally low affinity for 1,25-(OH)2D may explain this patients resistance to 1,25-(OH)2D. Therapy to maintain extremely high serum 1,25- (OH)2D3 concentrations markedly improved mineral homoeostasis, but did not affect the hair disorder.