The impact of iron supplementation and malaria chemoprophylaxis was investigated in a double-blind, placebo-controlled study involving 832 infants born in a malaria-hyperendemic area of Tanzania in 1995. Infants were randomly assigned to receive daily oral iron (2 mg/kg) and weekly Deltaprim (3-125 mg pyrimethamine plus 25 mg dapsone), daily iron plus weekly placebo, or daily and weekly placebo. Daily supplementation was provided from 8 to 24 weeks of age, while weekly chemoprophylaxis was given from 8 to 48 weeks. The 2 groups that received iron supplementation had a lower frequency of severe anemia (packed cell volume under 25%) than those who received placebo (0.62 versus 0.87 cases per person-year; protective efficacy, 28.8%), but iron supplementation did not have a significant effect on malaria incidence (0.87 versus 1.00 cases per person-year; protective efficacy, 12.8%). Infants who received malaria prophylaxis had lower frequencies of both severe anemia (0.45 versus 1.04 episodes per person-year; protective efficacy, 57.3%) and malaria (0.53 versus 1.43 episodes per person-year; protective efficacy, 60.5%) than those who received placebo. However, after the end of the intervention period, children who had received malaria prophylaxis had higher rates of severe anemia and malaria than those in the non-chemoprophylaxis groups (relative risks, 2.2 and 1.8, respectively). These findings indicate that malaria chemoprophylaxis during the first year of life can impair the development of natural immunity, while iron supplementation effectively prevents severe anemia without increasing susceptibility to malaria.