Abstract Pseudovitamin D–defiency rickets (PDDR) is an autosomal recessive disorder characterized by hypocalcemia, rickets (which are resistant to treatment with vitamin D), and low or undetectable serum levels of 1,25-dihydroxyvitamin D (1,25(OH)2D). The symptoms are corrected with 1,25(OH)2D treatment, and the disease is now believed to result from a defect in the cytochrome P450 component (P450c1; CYP27B1) of the renal 25-hydroxyvitamin D-1α-hydroxylase (1-OHase). We have studied genomic DNA from three families with PDDR and have identified the same homozygous mutation in the P450c1 gene in two of the index cases, causing a frameshift in exon 8, resulting in a premature stop codon in the heme-binding domain. The two cases in the third kindred were compound heterozygotes with missense mutations in exons 6 and 9. We have also identified a C/T polymorphism in intron 6 of the P450c1 genomic DNA. Interferon γ–inducible 1-OHase activity in blood-derived macrophages was shown by 1,25(OH)2D synthesis in all control cells tested (37–184 fmol/h/106 cells) and those from the PDDR family parents (34–116 fmol/h/106 cells) but was totally absent from the patients' cells, indicating a defect in their macrophage 1-OHase, similar to the presumed renal defect. The assumption of similarity between the renal and macrophage P450c1 was supported by our ability to clone a 514 bp sequence, including the heme-binding region of the macrophage P450c1 cDNA from controls, which was identical to that published for both the renal and keratinocyte P450c1 cDNAs.