Serotonin syndrome, an adverse drug reaction, is a consequence of excess serotonergic agonism of central nervous system receptors and peripheral serotonergic receptors. Serotonin syndrome has been associated with large numbers of drugs and drug combinations, and serotonin-norepinephrine reuptake inhibitor-induced serotonin syndrome is rare. It is often described as a sign of excess serotonin ranging from tremor in mild cases to delirium, neuromuscular rigidity, and hyperthermia in life-threat-ening cases. Diagnosis is based on the symptoms and patient's history, and several diagnostic criteria have been developed. We experienced a rare case of fibromyalgia accompanied by tremor, hyperreflexia, spontaneous clonus, muscle rigidity, and diaphoresis after 10 days of single use of duloxetine 30 mg. Only one case of serotonin syndrome resulting from administration of duloxetine has been reported in Korea, however that case resulted from co-administration of fluoxetine. We report here on this case along with a review of the relevant literature.