Nine premature infants developed radiographic and biochemical rickets at a mean ±SD of 12.6±2.8 weeks of age. Serum 25-hydroxyvitamin D concentrations were all low, with a mean of <3.6±2.1 ng/ml. The mean average daily intake of vitamin D since birth had been 300±181 IU, and the mean average daily intake during the week of diagnosis was 587±313 IU. All of the infants were extremely premature (mean weight 948±153 gm, mean gestation 27.7±1.1 weeks), and were being fed either a low-calcium "human milk-like" formula or a soy formula. It is postulated that low-calcium intake may have increased 25-OHD utilization in the face of a decreased ability of the extremely premature infant to produce 25-OHD. Because of multiple factors leading to both decreased production and possible increased utilization of 25-OHD, such infants have an increased requirement for vitamin D to maintain normal serum 25-OHD concentrations, and daily intakes of at least 400 IU vitamin D orally must be assured. Serum 25-OHD measurements and radiographs may be important in following infnats at risk.