While a large body of literature depicting relationships between depression or stress and immunity exists, few such studies have dealt with children, and none investigated myeloid cell-derived immunity. We investigated both phagocytosis and bactericidal activity against Staphylococcus aureus in children with major depressive disorder (MDD). We found that both MDD and stress influence the bactericidal but not the phagocytic activity of polymorphonuclear leukocytes. The data support the existence of psychobiologic effects in children and suggest possible mechanisms by which depression and stress may affect health.