Hypersensitivity pneumonitis (HP) can be induced by exposure to indoor molds contaminating humidifiers and heating or ventilation systems. A 54-year-old woman with dyspnea, cough, chest pain, and fever was seen in the emergency room. A chest radiograph revealed interstitial infiltrates and blood tests showed leukocytosis with neutrophilia and severe hypoxemia. A diagnosis of HP was made by a combination of clinical, radiologic, physiologic, and immunologic studies. Trichoderma viride was isolated in cultures of water samples from an ultrasonic humidifier installed in the patient's home a year earlier. Precipitating immunoglobulin G antibodies to T viride were detected in the patient's serum by enzyme-linked immunosorbent assay. The patient remained symptom free after the humidifier was removed from her home. Our findings strongly suggest that the patient developed HP due to T viride from the humidifier. To our knowledge, this is the first report of such a case.