A 50-year-old woman had her first episode of idiopathic thrombocytopenic purpura (ITP) at the age of 19. She was first treated with prednisolone (PSL); however, it was found to be ineffective. She was then treated with vincristine, immunoglobulin preparations, danazol, cyclosporine, and splenectomy for refractory ITP. Lastly, she was administered PSL and diaphenylsulfone (DDS). She had felt dyspnea at night, about one month before beginning to take DDS. Approximately 2 months after beginning to take DDS, she complained of dyspnea, orthopnea, and wheezing and was admitted to our hospital with a diagnosis of bronchial asthma. Oxygen therapy and PSL, aminophylline, and salbutamol inhalation were administered to the patient, but she still complained of dyspnea, and cyanosis and oxygen desaturation on the SpO₂ profile were observed. After her platelet count had increased to sufficient levels, a blood gas analysis was performed. It revealed a difference between SpO₂ (92%; O₂ at 3 L/min) and SaO₂ (97.6%) or PaO₂ (110.5 Torr) with 4.0% of methemoglobinemia. We decided to discontinue DDS. Her symptoms and the difference between SpO₂ and SaO₂ improved immediately. Moreover, the methemoglobin level in the blood was reduced to a normal level. Respiratory physicians must be made aware that DDS can induce methemoglobinemia.