Many diseases, mainly infectious and inhalational, have been associated with the use of hot tubs. "Hot tub lung" is a recently described disease entity associated with Mycobacterium avium complex (MAC) and is thought to be either an infection or a hypersensitivity pneumonitis. We describe 2 patients with progressively worsening respiratory symptoms and pulmonary function, along with diffuse radiographic changes consisting primarily of ground-glass opacities. Treatment with corticosteroids, based on lung biopsies suggesting sarcoidosis in 1 patient and eosinophilic bronchiolitis in the other, resulted in little improvement with both patients experiencing respiratory failure. Both patients continued regular and continued hot tub use despite ongoing respiratory difficulties, and MAC was identified in the hot tub water and/or lung tissue from each patient. Discontinuation of hot tub use, without antimycobacterial therapy, led to prompt improvement in symptoms, pulmonary function, and radiographic abnormalities, strongly supporting a diagnosis of hypersensitivity pneumonitis. Hypersensitivity to MAC, rather than an infection, is the likely underlying mechanism in these 2 cases of hot tub lung.