Allergic alveolitis (hypersensitivity pneumonitis) developed in two patients who were exposed to home humidifiers contaminated by thermophilic actinomycetes. Diagnosis was difficult because severe dyspnea was chronic and not easily associated with specific environmental exposure. Furthermore, chest roentgenograms were normal for long periods despite severe physiologic abnormalities. After several years of disabling symptoms, open lung biopsy findings suggested allergic alveolitis rather than sarcoidosis because a mononuclear interstitial infiltrate overshadowed the granulomas. Also, plasma cells were prominent and there was an intense bronchiolitis. Cultures of the humidifier water grew thermophilic organisms to which the patient's sera formed precipitins. Both patients experienced notable subjective and objective improvement following removal of the contaminated appliances. The ultrastructure of alveolar macrophages and basement membranes was similar to that described in animal models, suggesting cell-mediated hypersensitivity in the patients.