To assess the adequacy of different dosages of neonatal vitamin D, 25-hydroxyvitamin D serum concentrations were longitudinally monitored in 27 low-birth-weight and 25 full-term well infants from birth to 16 weeks after delivery. The infants were randomly assigned to receive either 10 micrograms/d (400 IU/d) or 20 micrograms/d (800 IU/d) of vitamin D or 0.85 or 1.5 micrograms/d of 25-hydroxyvitamin D3. In each infant who received 10 or 20 micrograms/d of vitamin D 25-hydroxyvitamin D, serum concentrations greater than 20 ng/mL were maintained, with some low-birth-weight infants reaching 60-ng/mL concentrations. Similarly, in the low-birth-weight infants receiving 1.5 and 0.85 micrograms/d of 25-hydroxyvitamin D3, serum 25-hydroxyvitamin D levels greater than 12 ng/mL were maintained. In the full-term infants who received 1.5 micrograms/d of 25-hydroxyvitamin D3, serum 25-hydroxyvitamin D concentrations of greater than 12 ng/mL were maintained, but in those who received 0.85 micrograms/d, serum 25-hydroxyvitamin D concentrations of 10 ng/mL could not be maintained. These vitamin D status data document that 10 micrograms (400 IU) of vitamin D represents a sufficient daily intake for both premature and full-term well infants. These data also indicate that while as little as 0.85 micrograms/d of 25-hydroxyvitamin D3 may facilitate vitamin D sufficiency in low-birth-weight neonates, it does not do so in full-term infants.