A controlled trial of iron prophylaxis (3 ml intramuscular iron dextran) to two-month-old infants was carried out on the north coast of Papua New Guinea where there is high transmission of malaria. The initial hypothesis was that iron deficiency increased susceptibility to infections and thus iron supplementation in a situation of actual or potential iron deficiency would diminish this susceptibility. Findings detailed elsewhere indicate that the placebo control group became relatively iron deficient and that the iron dextran group had adequate iron stores and a higher mean haemoglobin; however, prevalence of malaria recorded in the field was higher in the iron dextran group. Analysis of field and hospital infectious morbidity in the trial indicated a deleterious effect of iron dextran for all causes and for respiratory infections (the main single reason for admission). Total duration of stay in hospital was significantly increased in the iron dextran group. Analysis of other factors showed a deleterious effect of low weight for height at the start of the trial; a significant positive correlation between birth haemoglobin and hospital morbidity rates and a positive interaction between haemoglobin and iron dextran on hospital morbidity. A possible association between malarial experience and other infectious morbidity is discussed.