The manufacturer of deprenyl (selegeline; Eldepryl) (Somerset Pharmaceuticals, Tampa, FL) recently advised physicians to avoid prescribing the drug in combination with an antidepressant because of potentially serious CNS toxicity that may represent the serotonin syndrome. Manifestations of the serotonin syndrome vary but may include changes in mental status and motor and autonomic function. To better estimate the frequency of the serotonin syndrome in patients with Parkinson's disease (PD) treated with deprenyl and an antidepressant, we surveyed all investigators in the Parkinson Study Group. Based on estimates provided by the 47 investigators (75%) who responded, 4,568 patients were treated with the combination of deprenyl and an antidepressant medication. Eleven patients (0.24%) were reported to have experienced symptoms possibly consistent with the serotonin syndrome. Only two patients (0.04%) experienced symptoms considered to be serious. No deaths were reported. We also reviewed all published case reports and adverse experiences reported to the U.S. Food and Drug Administration and the manufacturer of Eldepryl. Available information indicates that serious adverse experiences resulting from the combined use of deprenyl and an antidepressant medication in patients with PD are quite rare and that the frequency of the true "serotonin syndrome" is even rarer.