This chapter represents an adaptation of a previously published report in the New England Journal of Medicine. Demonstration of the macrophage-activating capabilities of interferon gamma in patients with malignant diseases provided the initial impetus for study of interferon gamma administration in disease states characterized by deficient macrophage activity. Chronic granulomatous disease appears to be the prototypical example of such a disease, as such, may serve as a paradigm for the anti-infective application of interferon gamma. Chronic granulomatous disease is a group of inherited disorders of immune function characterized by recurrent pyogenic infections which usually present early in life and may lead to death in childhood. A considerable amount of evidence has been generated establishing a role for interferon gamma as an important macrophage-activating factor, a property shared to only a minimal extent with other lymphokines. The primary endpoint of this investigation was time to serious infection as defined by a clinical event requiring hospitalization and the administration of parenteral antibiotics.