A longitudinal study of 300 infants from birth to 1 year of age was carried out in Kingston, Jamaica. Haemoglobin levels were estimated 7 times during the year and serum iron and total iron binding capacity once. Hb electrophoresis was performed. In singleton children with Hb genotype AA, AS, or AC, and of birthweight 2·5 kg or over, Hb levels were low after 3 months of age. These low levels were associated with iron deficiency, which was probably due to poor iron stores at birth followed by poor iron intake or absorption. It was not possible to determine whether folic acid deficiency or protein deficiency was also important. Hb levels varied with age, socioeconomic class, birthweight, sex, and rate of weight gain. The growth and health of 4 girls with homozygous sickle cell disease is mentioned.