In this study the effects of supplementation of iron and zinc, alone or combined, on iron status, zinc status and growth in Indonesian infants is investigated. Micronutrient deficiencies are prevalent in infants in developing countries, and deficiencies often coexist; thus, combined supplementation is an attractive strategy. However, little is known about interactions between micronutrients. In a randomized, double-blind, placebo-controlled supplementation trial, 478 infants, 4 mo of age, were supplemented for 6 mo with iron (10 mg/d), zinc (10 mg/d), iron + zinc (10 mg of each/d) or placebo. Anthropometry was assessed monthly, and micronutrient status was assessed at the end of supplementation. Supplementation significantly reduced the prevalence of anemia, iron deficiency anemia and zinc deficiency. Iron supplementation did not negatively affect plasma zinc concentrations, and zinc supplementation did not increase the prevalence of anemia or iron deficiency anemia. However, iron supplementation combined with zinc was less effective than iron supplementation alone in reducing the prevalence of anemia (20% vs. 38% reduction) and in increasing hemoglobin and plasma ferritin concentrations. There were no differences among the groups in growth. The growth of all groups was insufficient to maintain the same Z-scores for height for age and weight for height. There is a high prevalence of deficiencies of iron and zinc in these infants, which can be overcome safely and effectively by supplementation of iron and zinc combined. However, overcoming these deficiencies is not sufficient to improve growth performance in these infants.