SummaryThe plasma concentrations of 25-hydroxyvitamin D (OHD), 1,25-(OH)2D and 24,25-(OH)2D were determined in 28 healthy premature infants(median gestational age 33, range 28-36 wk; and median birth weight 1880, range 900-2350 g) during the first 5-10 wk of life,and in a reference group of 17 young adults. The infants received a vitamin D supplement of 500 IU/d and a diet low in calcium (Ca)and phophorus (P) compared with that of corresponding intrauterine accretion rates.The median 25-OHD concentration increased from 11 (range 6-30) ng/ml at 1 d to 27 (range, 15-41) ng/ml by 5-10 wk of age (P<0.01). 1,25-(OH)2D concentrations at age 1 d were similar to the adult levels (median 37, range 8-64 versus 35, range 18-58 pg/ml), but increased significantly within 1 wk to 48 (26-156) pg/ml (P=0.01), and between 1 and 3-4wk of age to 104 (58-203) pg/ml (P< 0.01). The levels at 5-10 wk were similar to the 3-4 wk value. 24,25-(OH)2D concentrations were persistently low compared with the adult levels (medians 0.4- 0.5, range <0.3-2.1 versus 1.7, range 0.4-2.0 ng/ml, P<0.01). The relative concentrations, expressed as the ratio of 24,25- (OH)2D to 25-OHD, were comparable to those of the adults at birth, but decreased significantly within 2 wk. The data demonstrate that healthypremature infants can produce high plasma levels of 1,25-(OH)2D.