Madang district was selected for a longitudinal study of the effects of iron prophylaxis on infectious morbidity in infancy and the topography, climate, domicile, ethnology, demography, disease patterns, nutrition and health services of the district are described. The area has a tropical, humid climate and a mixed economy. Pneumonia was the main killing disease at all ages, and malaria was endemic. A base hospital and well organized maternal and child health services ensured that morbidity surveillance would be optimal. Pilot haematological surveys confirmed a high incidence of anaemia in infancy. Mean haemoglobin between nine and 52 weeks of age was 8.6 g/dl. Results suggested that malaria and iron deficiency were important causes of this anaemia.