Serotonin syndrome is characterized by varied degrees of cognitive, autonomic, and neuromuscular dysfunction and can only be produced by drug therapy that increases central nervous system serotonin neurotransmission. Information gained from a retrospective review of 127 cases of serotonin syndrome is presented. It is not uncommon for severe cases of serotonin syndrome to be confused with neuroleptic malignant syndrome. Treatment is mainly supportive, but specific pharmacologic therapy with serotonin antagonists may be potentially beneficial.