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Exposure to microbial aerosols from indoor hot tubs and therapy pools can cause granulomatous lung disease. However, the clinical presentation, causative antigen, and factors affecting outcome remain poorly understood. We performed a retrospective case series of all patients seen at our institution from 1994 to 2002 with a diagnosis of hypersensitivity pneumonitis attributed to hot tub or therapy pool exposure (hot tub lung). We identified 27 immunocompetent patients with hot tub lung. The patients had symptoms accompanied by pulmonary function and high resolution chest computed tomography abnormalities suggestive for hypersensitivity pneumonitis. Nontuberculous mycobacteria (NTM) (mainly Mycobacterium avium complex) were cultured from most of the patients. Environmental sampling performed at a third of the implicated sites showed high concentrations of NTM in both air and water samples. Removal from exposure combined with oral corticosteroids and, in a few cases, NTM antibiotics, led to significant improvement or resolution of illness in all affected, although mild residual functional abnormalities persisted in the majority. Exposure to aerosols containing NTM from indoor warm water pools and spas can cause a granulomatous lung disease with features of hypersensitivity pneumonitis. Recognition of these environmental exposure sources and appropriate management leads to a generally favorable prognosis.