210 JW Yoon, et al. • Vitamin D deficiency and iron deficiency anemia factor for VDD (P<0.001)11). We also found the positive correlation between the younger age than 5 years with VDD (P<0.001). We found that seasonal variation was an independent predictor of VDD in infants with IDA, in agreement with previous studies15-18). In particular, we found that VDD was more common and the level of serum 25OHD was significantly lower in spring/winter than summer/autumn. However, serum calcium, phosphorus, and ALP did not exhibit significant seasonal variations. Our multivariable logistic analysis indicated that seasonal variation in 25OHD was the only significant independent risk factor for VDD. Therefore we suggest the seasonal variation is the main risk factor of VDD in children with IDA. Interestingly serum Hb and ferritin concentrations were higher in winter than in summer, although these differences were not significant. A previous study of Nepali pregnant women reported that the prevalence of anemia was higher in summer/autumn than in winter/spring (P<0.001) and that Hb was lower during the monsoon period than in the winter34,35). We cannot fully reconcile these results with our own results, but they may be different due to differences in patient characteristics, economic status, and health care systems and status. Most of our subjects received breastfeeding and were younger than 24 months, so we assumed that maternal factors play a role in the Hb and ferritin levels of the children. This subject warrants further study. The limitations of our study are we restricted our patient population to children with IDA, and only a small number of children were over 2 years old. In addition, we did not examine a control group of healthy children, so our results on the prevalence of VDD in IDA cannot be generalized to other populations. 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