A 65-year-old man was admitted due to low grade fever, dry coughing, and dyspnea on exertion. The chest radiograph and CT scan showed diffuse ground glass opacities and small nodules in the both lung fields resulting in a diagnosis of severe interstitial pneumonia. Conservative treatment with antibiotics and bronchodilators decreased the symptoms, but the dyspnea and cough reappeared when he returned home. An inspection of his house revealed the presence of fungi under the wallpaper. His symptoms disappeared completely after these were removed. His clinical course raised the suspicion of hypersensitivity pneumonitis and these fungi believed to be the cause of hypersensitivity pneumonitis. The histological findings of a lung specimen by video-assisted thorachoscopy were compatible with hypersensitivity pneumonitis. The fungi were identified as Alternaria.