A 38-year-old woman was admitted to the Iiyama Red Cross Hospital with a non-productive cough. She had been working with Strophariaceae cultures for 4 years, and in January 1999 a cough had developed. On admission, hypoxemia was noted. The serum level of lactate dehydrogenase was slightly elevated. Chest radiography and CT scanning showed diffuse ground glass opacity and small centrilobular nodules in both lower lobes. In the bronchoalveolar lavage fluid, the total cell count was normal, the CD4/CD8 ratio was reduced, and the lymphocyte fraction was very high. A transbronchial lung biopsy revealed alveolitis with lymphocytic infiltration and organizing changes like Masson bodies. The lymphocyte stimulation test was positive for Strophariaceae. Hypersensitivity pneumonitis caused by Strophariaceae was diagnosed. Prednisolone was administered because of persistent symptoms and prolonged abnormal chest radiography findings. These abnormalities disappeared 2 weeks after prednisolone treatment was initiated. There is one previous report of hypersensitivity pneumonitis caused by Strophariaceae. The symptoms are coughing and sputum production as in the common cold. The occupational history must be carefully considered in making a diagnosis.