An 8-year-old girl visited the emergency room with perioral cyanosis and dyspnea, which had developed 20 hrs prior to the hospital visit. She had taken a Chinese herbal medication for 3 days prior to the onset of the symptoms. A co-oximeter examination revealed a methemoglobin level of 23.7%. An intravenous infusion of methylene blue was administered. Chemical analysis of the herbal medication revealed an ammonia (NH3) level of 239.41 mg/L. More studies are needed on the correlation between methemoglobinemia and the components of Chinese herbal medicines.