We have demonstrated impaired intestinal absorption of calcium in a child with active vitamin D dependent rickets (hereditary pseudovitamin D deficiency rickets) at a time when the patient had normal anti-rachitic activity in her serum. Calcium absorption improved greatly in response to vitamin D, administered in the massive dosage that was necessary to heal the rachitic lesions. Phosphorus absorption may have been slightly impaired in the same patient, but no other absorptive defect was found. We studied intestinal function in five additional patients after they had been placed on vitamin D therapy. No abnormalities were found. In these treated patients, calcium absorption was not measured. Duodenal mucosa studied by light and electron microscopy was normal in all patients. Future investigation of intestinal transport of calcium in these patients should help to explain the pathogenesis of this disease.

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