OBJECTIVE To compare the incidence of anemia between infants who received exclusive breast feeding (EBF) up to the age of 6 months and those who did not. METHODS A cross sectional survey was conducted to examine the physical growth, incidence of anemia, mean hematocrit (Hct) level and other red cell indices among 6-month-old infants visiting a well-child clinic at QSNICH and Ramathibodi Hospital. RESULTS 63 infants were included in the present study: 24 (38%) EBF group; and 39 (62%) non-EBF group. Mean Hct of EBF group was 34.5 +/- 1.7 and Non-EBF group was 35.1 +/- 2.3% respectively. The prevalence of anemia in the EBF group was 4.2%, non-EBF group was 12.8%. Even though the findings were not statistically significant, the authors found that the red blood cell indices in an anemic infant in the EBF group were not caused by iron deficiency, while the 4 of 5 anemic infants in the Non-EBF group were related to iron deficiency anemia (80%). Other growth parameters were compared between the two groups. CONCLUSIONS The present study did not show statistical difference regarding the different hematocrit and other red blood cell indices including other fundamental background but the prevalence of anemia appeared to be higher among Non-EBF compared to EBF up to 6 months. The cause of anemia was not related to iron deficiency anemia in the EBF group compared to those higher and related to iron deficiency anemia in the Non-EBF group. METHODS EBF up to 6 months should be advocated given its substantial health benefit and does not increase the risk of anemia. Nevertheless, it is essential that mothers receive adequate nutritional supplement including iron during pregnancy and lactation period.