This study aimed to describe the evolution of hemoglobin concentration considering the following factors: birth weight, growth rate, and food intake. The cross-sectional study focused on infants (<1 year of age) whose growth and development were monitored by public health services in cities located in the five geographic regions of Brazil. Some 51.7% of the children aged 6 to 12 months presented anemia. Estimation of [Hb] concentration by a theoretical equation suggested that endogenous iron is able to maintain normal Hb levels in the first three months of life. Prevalence of reduced Hb was higher in low birth weight infants. Growth rate, verified by the difference between present weight and birth weight in Z scores, was not the relevant factor for hemoglobin concentration. The relevance of breastfeeding was evident in the initial months of life. However, complementary foods did not influence infant Hb concentration. The high prevalence of low Hb levels calls attention to the need for programs to control anemia.