A 54-year old male patient who was suffering from bipolar I disorder for 19 years and was admitted to the National Bugok Mental Hospital due to a depressive episode, was referred to the Kosin University Gospel Hospital. On arrival at the emergency room, he had confused mentality with disorientation, memory impairment, hypomania, marked anxiety and hyperirritability. The change of neuromuscular activity such as ataxia, gait disturbance, tremor, shivering, myoclonus and epileptic seizures was also shown. In addition, the symptoms and signs of autonomic instability including diaphoresis, tachycardia, hypotension, fever and facial flushing were noticed. The above symptoms developed after the administration of sertraline successive to the discontinuation of fluoxetine without any washout period. The degree of severity seemed to be severe because he had epileptic seizures, fever and hypotension. He was recovered from the severe serotonin syndrome by the supportive symptomatic treatment with sodium valproate, clonazepam, lorazepam and cyproheptadine after cessation of the selective serotonin reuptake inhibitors during hospitalization. Therefore, this rare case of severe serotonin syndrome was reported and related literatures were also reviewed.