A 13-year-old girl with total alopecia who in infancy had rickets unresponsive to large doses of vitamin D2 is described. She had profound hypocalcæmia which was resistant to treatment with high doses of dihydrotachysterol, 1α-hydroxycholecalciferol, and 1,25-dihydroxycholecalciferol. Serum concentrations of 25-hydroxyvitamin D were normal but those of 1,25-dihydroxycholecalciferol were markedly raised (674 and 745 pg/ml). In addition, 24,25-dihydroxyvitamin D was undetectable in serum. Administration of synthetic 24,25-dihydroxycholecalciferol was followed by normocalcæmia which persisted long after treatment was stopped. Her sister, who died at the age of 10 months, also had had total alopecia, rickets, and hypocalcæmia resistant to vitamin-D2 therapy. In this familial syndrome there seems to be end-organ resistance to the action of 1,25-dihydroxycholecalciferol, possibly as a result of changes at the receptor sites.