PIP: The hypothesis that breastfed infants in Beijing, China, have low vitamin D status and that sunshine exposure increases serum 25-hydroxyvitamin D (25-OHD) concentrations was tested in a randomized prospective study involving 42 healthy infants 1-8 months of age. 24 controls were exposed to the usual amount of sunshine, while the 18 cases were provided with 2 hours of sunshine/day. All infants were being breastfed without vitamin D supplementation. Serum 25-OHD concentrations at entry into the study ranged from 3-61 ng/ml and increased with age. 3 infants in each group were diagnosed as having rickets. Serum 25-OHD concentrations did not change significantly during the 2-month study period among controls, while serum 25-OHD values increased significantly in the experimental group. Final infant serum 25-OHD concentrations correlated with ultraviolet exposure scores (p.001). The estimate of ultraviolet score needed to maintain a serum 25-OHD of 11 ng/ml was 2.4 (24 minutes/day with only the face uncovered). Rickets has been reported in as many as 18% of Chinese infants and seems to especially affect those 2-4 months of age. The fact that not all infants with rickets in this study had low 25-OHD concentrations suggests 2 possibilities: 1) not all rickets is necessarily related to a vitamin D deficiency, or 2) serum 25-OHD concentrations are not the best indicator of vitamin D status. The low serum 25-OHD concentrations in many Chinese infants may be related to low maternal milk vitamin D content resulting from the lack of vitamin D food fortification. Overall, these results suggest that the diet of breastfed Chinese infants should be supplemented with orally administered vitamin D.