To determine whether amounts of vitamin D lower than recommended doses are effective in preventing rickets, 256 term infants from two northern and two southern cities in China were studied in a randomized trial of vitamin D supplementation (100, 200, or 400 IU/day) during the first 6 months of life. Cord blood and 6-month blood samples were collected and radiographs were obtained at 3 to 5 days and at 6 months of age. Cord serum 25-hydroxyvitamin D concentrations were lower in the north than in the south (5 vs 14 ng/ml (12.5 vs 35.0 nmol/L); p less than 0.01). Wrist ossification centers were less likely to be present at birth in the northern children than in the southern children (p = 0.009) and were more likely to be present in infants born in the fall who had higher cord serum concentrations of 25-hydroxyvitamin D (p = 0.04). Serum 25-hydroxyvitamin D concentrations were lower in northern children 6 months of age than in southern children (p = 0.005) and were higher with an increasing supplemental dosage of vitamin D (p less than 0.001), particularly in infants in the north. None of the infants had rickets at 6 months of age. Because of the low serum 25-hydroxyvitamin D concentrations, especially among infants in the north, it may be prudent to supplement the diet with vitamin D at a dose of 400 IU/day.