We report the case of a 20-year-old female with disseminated <i>Mycobacterium avium</i> disease involving bones, lungs and brain. She was completely healthy up until the present illness and had been vaccinated with BCG in infancy without complications. Mycobacteriosis progressed in spite of treatment with antituberculous drugs and was controlled only after addition of interferon-γ subcutaneously. A homozygous hypomorphic I87T mutation was found in the gene encoding the ligand-binding chain of the IFN-γ receptor (IFNγR1). This mutation is the only known recessive hypomorphic lesion in <i>IFNGR1</i> and had been reported before in only 1 child with curable BCG infection and his sibling with primary tuberculosis. Our report illustrates the clinical heterogeneity of patients sharing exactly the same form of partial recessive IFNγR1 deficiency. A diagnosis of partial recessive IFNγR1 deficiency should be contemplated in adults with unexplained environmental mycobacterial diseases.