RATIONALE: The factors predisposing towards the development of pulmonary non-tuberculous mycobacterial disease (pNTM) and influencing disease progression remain unclear. Impaired immune responses have been reported in individuals with pNTM but data are limited and inconsistent. OBJECTIVES: To use gene expression profiling to examine the host response to pNTM. METHODS: Microarray analysis of whole blood gene expression was performed on 25 subjects with pNTM and 27 uninfected controls with respiratory disease. Gene expression results were compared to phenotypic variables and survival data. MEASUREMENTS AND MAIN RESULTS: Compared with uninfected controls, pNTM was associated with down-regulation of 213 transcripts enriched for terms related to T cell signalling including IFNG. Reduced IFNG expression was associated with more severe CT changes and impaired lung function. Mortality was associated with the expression of transcripts related to the innate immune response and inflammation, whereas transcripts related to T and B cell function were associated with improved survival. CONCLUSIONS: These findings suggest that pNTM is associated with an aberrant immune response which may reflect an underlying propensity to infection, or result from NTM infection itself. There were important differences in the immune response associated with survival and mortality in pNTM.