We report a case of a male patient who underwent splenectomy for idiopathic thrombocytopenic purpura. Oxygen saturation was 92% by pulse oximetry and cyanosis was apparent during the perioperative period. Methemoglobin (metHb) levels were 10.4%. After removal of the tracheal tube, intravenous methylene blue (I mg/kg) was administered because of persistently low SPO2 levels (87%). During preanesthetic evaluation, the patient did not inform the anesthesiologist that he had been taking dapsone.736377379