The 2005 National Nutrition Survey of Colombia reported that 38% of schoolchildren were anemic, yet the rate of iron deficiency was 3.6%. To determine if other micronutrients may be responsible for anemia, we examined hemoglobin (Hb) concentrations in relation to serum ferritin, vitamin B12, and vitamin A, and erythrocyte folate (eFol) in 2812 children (5–12 y) from Bogota, Colombia. Mean Hb concentration was 145 (SD, 12) g/L; 4% of children were anemic (Hb <127 g/L) while 3% had low ferritin (<15 μg/L). In multivariate analysis, Hb increased with child's age, mother's age, household's socioeconomic stratum, and family income. Low ferritin was related to 3.6 g/L lower Hb (95%CI=−6.0, −1.3). There was an inverse trend in Hb by quartiles of eFol; the adjusted Hb difference between the highest and lowest eFol quartiles was −6.0 g/L (95%CI=−7.2, −4.9, p for trend <0.0001). This difference was greatest among children with B12 <148 pmol/L (−9.6 g/L), followed by children with B12 148–221 pmol/L (−8.0 g/L), and smallest in children with B12 >221 pmol/L (−5.6 g/L); p for interaction = 0.04. The impact of improving vitamin B12 status on this unexpected inverse relation between folate and hemoglobin should be examined in populations where foodstuffs are fortified with folate alone. Funding: Harvard DRCLAS, Bogota's Secretary of Education, Colombian National Institute of Health, National University of Colombia