Increasing numbers of patients have presented with a hypersensitivity pneumonitis-type course in association with hot tub exposure. Mycobacterium avium complex (MAC) organisms have been isolated from both patient specimens and hot tub water with matching fingerprints by restricted fragment length polymorphism and electrophoresis when performed. Review of the clinical, microbiologic, and radiographic presentations of 9 patients to the Mayo Clinic with this diagnosis are compared with 32 patients in the published literature. The diagnosis, treatment, and prognosis of MAC hot tub lung are reviewed.