PURPOSE: The Large amounts of whole cow' s milk intake, just followed by prolonged breast milk feeding may induce a severe deficit of iron. We performed this study to investigate the severity of iron deficiency in these subjects. METHODS: Seventy-one subjects (12 to 48 months of age) were classified according to history of prolonged breast milk feeding and intake of large amounts of whole cow' s milk. Prolonged breast milk feeding and large amounts cow' s milk intake subjects were assigned as group A (n=15), the large amounts of cow's milk intake subjects as group B (n=15), the prolonged breast milk feeding subjects as group C (n=24) and the normal control subjects as Group D (n=17). The hematologic and iron status were evaluated and compared between each group. RESULTS: Although the mean of group A was lower than the mean of group B and C, the level of hemoglobin of group A was significantly lower than group D (P<0.01). The level of the ferritin of group A was significantly lower than group B, C (P<0.05, respectively) and group D (P<0.01). Of the 15 study subjects of group A, 14 (93.3%) had microcytic hypochromic anemia and 7 (46.7%) revealed that serum ferritin levels were at 1.0ng/dL, indicating severe deficit of storage iron. CONCLUSION: The intake of large amounts of whole cow' s milk, just followed by prolonged breast milk feeding induced iron deficiency anemia and a severe deficit of storage iron. It is necessary to inform parents to avoid irreversible recognition impairment and behavioral change.