We describe a patient who presented with hypoxemia and diffuse bilateral pulmonary infiltrates. The diagnosis of eosinophilic pneumonia was confirmed by bronchoalveolar lavage and transbronchial lung biopsy. The remarkable characteristic was reappearance of the symptoms on the patient's return home, suggesting the existence of etiologic agents in his house. An environmental survey of the patient's house yielded Alternaria alternata. A high titer of anti-A. alternata antibody (IgG) was detected in his serum, and the inhalation bronchoprovocation test with A. alternata antigen was positive. This case indicates that A. alternata is a probable cause of eosinophilic pneumonia.