The treatment of choice for pseudo-vitamin D deficiency rickets (PDDR), caused by mutations in the 25-hydroxyvitamin D-1alpha-hydroxylase (CYP27B1; 1alpha-OHase) gene, is replacement therapy with 1,25(OH)2D3. We have previously engineered an animal model of PDDR by targeted inactivation of the 1alpha-OHase gene in mice. Replacement therapy was performed in this model. The 1alpha-OHase-/- mice and heterozygote controls were treated with 500 pg of 1,25(OH)2D/g body weight/day for 2 weeks, followed by 100 pg of 1,25(OH)2D3/g body weight/day for an additional 3 weeks before death at 8 weeks of age. Blood biochemistry analysis revealed that the rescue treatment corrected the hypocalcemia and secondary hyperparathyroidism. The daily injections of 1,25(OH)2D3 induced strong expression of CYP24, the 25-hydroxyvitamin D 24-hydroxylase gene. Bone histology and histomorphometry confirmed that the rickets and osteomalacia were cured. The rescue regimen also restored the biomechanical properties of the bone tissue within normal parameters. These results show that chronic treatment with the active 1,25(OH)2D3 metabolite is effective to rescue the PDDR phenotype of 1alpha-OHase mutant mice.