We report a case of pneumonitis induced by Seisin-renshi-in. A 62-year-old man began to complain of cough, dyspnea and fever 45 days after starting to take Seisin-renshi-in for benign prostate hypertrophy. Chest radiograph showed diffuse ground-glass shadows in the bilateral middle and lower lung fields. Chest CT showed diffuse ground-glass opacities in both lung fields. The serum KL-6 level was elevated. Arterial blood gas analysis revealed hypoxia. Lymphocyte stimulation test with peripheral blood lymphocytes for Seisin-renshi-in was positive. A mild increase of lymphocytes, neutrophils and eosinophils was observed in the cell population of bronchoalveolar lavage fluids. Transbronchial lung biopsy specimen showed lymphocytic infiltration into the alveolar septa, desquamative alveolar lining cells and fibrinous exudate in the alveolar spaces. On the basis of a diagnosis of Seisin-renshi-in-induced pneumonitis, steroid therapy was introduced. Three courses of steroid pulse therapy were required because of prolonged hypoxia. His respiratory condition then improved and predonisolone was tapered from 30 mg. We should be aware that over-the-counter drugs could be causal agents of severe pneumonitis.