Interferon-γ receptor 2 (IFN-γR2) deficiency is a rare primary immunodeficiency characterized by predisposition to infections with weakly virulent mycobacteria, such as environmental mycobacteria and BCG vaccines. We describe here two children with IFN-γR2 deficiency, from unrelated, consanguineous kindreds of Arab and Israeli descent. The first patient was a boy who died at the age of 4.5 years, from recurrent, disseminated disease caused by Mycobacterium simiae. His IFN-γR2 defect was autosomal recessive and complete. The second patient was a girl with multiple disseminated mycobacterial infections, including infection with M. simiae. She died at the age of 5 years, a short time after the transplantation of umbilical cord blood cells from an unrelated donor. Her IFN-γR2 defect was autosomal recessive and partial. Autosomal recessive IFN-γR2 deficiency is life-threatening, even in its partial form, and genetic diagnosis and familial counseling are therefore particularly important for this condition. These two cases are the first of IFN-γR2 deficiency associated with M. simiae infection to be described.