A 48-year-old woman had an 18-month history of malaise and chronic cough with intermittent episodes of fever, chills, and pneumonic infiltrates. Transbronchial biopsy findings were consistent with hypersensitivity pneumonitis. Cultures of fungus from a hot-tub room in her home were positive for Cladosporium species. Serum precipitins were weakly positive for Cladosporium cladosporioides. Removal of the patient from the home environment led to a resolution of symptoms within 1 week. Within 4 hours of re-exposure to the hot-tub room, symptoms and signs and changes in leukocyte count and spirometric values again occurred. Bronchial provocation with a commercial extract of C. cladosporioides led to a similar pattern 5 hours after the initial challenge. This case identifies a previously unreported etiologic agent and environmental site for hypersensitivity pneumonitis.