A previously healthy 13-year-old child who had disseminated Mycobacterium avium infection is described. Further history revealed disseminated mycobacterial infections in the patient's father and uncle, starting at 9 years old and 1 year old, respectively. Autosomal dominant interferon- gamma receptor mutation was subsequently identified. Clinical variability among the affected members of the same family is consistent with previous reports suggesting substantial variability in the clinical course of this disorder.