The third child of a previously healthy woman was delivered by caesarean section. Because of intraoperative blood loss, a blood transfusion was given after the delivery. The baby was breast-fed for 6 weeks. One unit of blood came from a male in whom the acquired immunodeficiency syndrome (AIDS) developed 13 months later. On recall, the mother proved to have lymphadenopathy, serum antibody to the AIDS virus, and a reduced T4/T8 ratio. The infant, who failed two thrive and had atopic eczema from 3 months, has likewise proved to have antibody to the AIDS virus. Since his mother was transfused after his birth, he is presumed to have been infected via breast milk or by way of some other form of close contact with his mother.