Three patients are described with anorexia nervosa in whom malnutrition was present with neutropenia and a granulocyte bactericidal degect. Their peripheral blood granulocytes were found to have a reduced rate of killing of Staphylococcus aureus and E. coli in vitro. The opsonic activity of the patients' sera towards Staphylococcus aureus was normal. One of these patients had recurrent episodes of infection which stopped after she had gained 13 kg in weight. Clinical recovery was associated with a return of granulocyte function to normal. It is concluded that granulocyte bactericidal capacity towards a variety of bacteria may be reduced in patients with anorexia nervosa who have malnutrition. This type of acquired granulocyte bactericidal deficiency appears to be reversible.