We report the case of a 44-year-old woman with disseminated Mycobacterium avium complex (MAC) infection involving multiple bone lesions despite a normal healthy status until 6 months previously. Because she was suspected to have acquired immunodeficiency, we tested interferon (IFN)-gamma production by peripheral blood mononuclear cells (PBMC) after phytohemagglutinin (PHA) or anti-CD3 stimulation, and found that these cells produced no, or undetectable, levels of IFN-gamma in the presence of the patient's plasma, but produced nearly normal levels of IFN-gamma in the presence of healthy donor plasma. Since the IgG fraction of the patient's plasma was capable of blocking in vitro responses to IFN-gamma, the cause of disseminated MAC infection in this case appeared to be anti-IFN-gamma autoantibodies. To reduce the titer of anti-IFN-gamma autoantibodies, the patient received intravenous immunoglobulin (IVIG). However, titer of autoantibodies changed little compared to that before IVIG administration. According to our literature search, this is only the second case of disseminated MAC infection associated with anti-IFN-gamma autoantibodies in Japan.