[Vitamin D1alpha-hydroxylase deficiency as the cause of severe rickets in a 1-year-old-old boy].

We describe a 15-month-old boy referred to the hospital because of delayed development of motor skills and growth retardation. Blood samples and X-rays of the wrists and knees revealed rickets. He was treated with oral calcium and vitamin D with modest clinical and biochemical effect. 1,25-dihydroxyvitamin D was undetectable in laboratory tests. Vitamin D1alpha-hydroxylase deficiency was suspected and confirmed by DNA analysis, which revealed a 7 bp duplication in exon 8 of the CYP27B1 gene. The treatment was changed to an activated formula of vitamin D, alphacalcidol, whereupon the clinical and biochemical symptoms rapidly improved.