We describe two brothers of consanguineous Pakistani parents who lived in Norway and had disseminated infections due to nontuberculous mycobacteria. The first boy developed clinical signs of disseminated BCG infection after vaccination. He was successfully treated with antimycobacterial agents. Two and one-half years later, he developed disseminated Mycobacterium avium complex infection and died at 6 years of age. The second boy, born 5 years after the death of his brother, did not receive BCG vaccine. At 2 years of age, he developed disseminated M. avium complex infection. Because he responded only partly to specific chemotherapy, empirical interferon γ treatment was added to the antimycobacterial regimen. After 2 years of combined therapy, his condition is stable. Studies of peripheral blood mononuclear cells from the second boy demonstrated reduced surface expression of the ligand binding chain of interferon γ receptor 1. This defect explains the increased susceptibility to mycobacterial disease in the two brothers.