A pregnant woman suffering from idiopathic hypoparathyroidism was treated with calcitriol [0.5-2 micrograms/day 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3)]. Her twin infants were delivered by cesarian section at 37 weeks of gestation. Laboratory investigations in the perinatal period disclosed: 1) normal serum calcium and phosphorus levels in the mother, 2) normal babies with no clinical or biochemical signs of hyperparathyroidism, 3) a low serum level of 25-hydroxyvitamin D despite a normal serum level of 1,25-(OH)2D in the mother, and 4) a low level of 25-hydroxyvitamin D and a high level of 1,25-(OH)2D in cord serum in both infants. It is suggested that calcitriol is an effective treatment of hypoparathyroidism during pregnancy and produces no ill effects on the baby.