A 47-year-old Japanese woman was admitted to our hospital with a 2-week history of dry cough and shortness of breath. She had been engaged in Enoki mushroom production for 22 years. Chest X-ray and chest computed tomography (CT) scan showed bilateral fine-nodular shadows and ground glass opacity. Bronchoalveolar lavage fluid demonstrated an increase of total cell counts with predominant lymphocytosis. Pathological specimens obtained by video-assisted thoracoscopic surgery revealed alveolitis and noncaseating granuloma with giant cells. Lymphocyte stimulation test showed positive responses with Enoki mushroom, culture medium, and Penicillium citrinum. On double immunodiffusion test, a precipitation line was observed between patient's serum and Penicillium citrinum antigen. She was found to have hypersensitivity pneumonitis caused by Penicillium citrinum. This is the first report of mushroom worker's lung caused by Penicillium citrinum.