objective: to present a review about the main determining factors of anemia in children under 5 years old. source of data: information was used from articles published in indexed national and international scientific journals, technical books and publications from international organizations. summary of the findings: anemia constitutes the world's nutritional problem of the greater magnitude, and children under 5 years old represent one of the highest risk population groups. as with any public health problem, the origin of anemia is multi-causal and thus in this article the attempt is to interpret its direct or indirect relation with possible determinant factors and the main concordant or discordant findings in epidemiological studies. among these factors are the social and economic conditions, the conditions of child health care, the child's nutritional state, the presence of morbidity, food consumption and biological factors. the role of the diet is emphasized with respect to the consumption and bioavailability of iron, and age of the child as the main determinants. conclusions: bearing in mind the magnitude of the problem and the breadth of its risk factors reviewed in this study, the implementation of urgent prevention and treatment measures for iron-deficiency anemia becomes necessary. it is important to stress that a single strategy may have little success if other measures are not taken simultaneously; the role of nutritional education being relevant, together with other implemented actions. children under 2 years old and children who live in rural and deprived areas should be priorities in programs to combat anemia.