Iron deficiency anemia (IDA) is considered to be the most prevalent micronutrient deficiency in the world. Estimates indicate that 1.2 billion people suffer mild to severe forms of anemia and that up to 46% of schoolchildren in developing countries are affected. In 2003, ENDSA, the national demographic and health survey of Bolivia showed that 60% of children under five and 72% of children under 2 years old were anemic. Micronutrient deficiency has been suggested to impair cell-mediated immunity. In particular, iron, zinc and vitamin A deficiencies have an impact on the immune system. In vitro and in vivo laboratory studies indicate a link between iron deficiency and impaired T-lymphocyte proliferation. The exact effects or mechanisms of iron deficiency on maturation and proliferation of T-lymphocytes in vivo are, however, not yet known. This study investigated the effects of iron on the maturation of T-lymphocytes in anemic but otherwise healthy schoolchildren (no apparent protein-energy deficiency or other morbidity). Anemic children of a poor peri-urban school of Cochabamba city, Bolivia, were given iron treatment for three consecutive months. We chose to look at CD1a+ lymphocytes, which are immature thymocytes. The proportions of CD1a+ lymphocytes in the peripheral circulation measured at baseline and after treatment were compared with a reference group of age-matched non-anemic children controls from the same school. The immunologic parameters, although improved, did not reach the proportions of the control group. Overall, the proportion of circulating immature T-lymphocytes decreased from 18.3% to 9.2% in the treated following iron supplementation in anemic children, compared with 3.4% in non-anemic children.