Recently various genetic defects in immunity mediated by interferon γ (IFN-γ) have been described, including mutations in the IFN-γ receptor 1 (IFN-γR1) and receptor 2 (IFN-γR2), signal transducer and activator of transcription 1 (STAT 1), and interleukin 12 receptor β 1 (IL-12Rβ1), and IL-12 p40 genes. These mutations are associated with the occurrence of severe infections with intracellular pathogens especially nontuberculous mycobacteria and vaccine-associated bacilli Calmette-Guérin (BCG). Here we report data on a previously healthy adult patient primarily presenting with severe infections with Burkholderia cocovenenans and subsequently Mycobacterium cheloneae. We found a strong inhibitory anti–IFN-γ activity in the patient's plasma and identified a highaffinity neutralizing anti–IFN-γ autoantibody. Unfortunately, the patient died due to severe sepsis before we knew the nature of the inhibitory activity. The application of alternative therapeutic approaches such as intravenous immunoglobulin or immunoadsorption may have been beneficial in this case. Screening for neutralizing anti–IFN-γ autoantibodies should supplement testing for IFN-γ and IL-12 pathway defects in patients with recurrent infections with intracellular pathogens, especially with nontuberculous mycobacteria.