Restless leg syndrome (RLS) is usually idiopathic but may occur in patients with Parkinson's disease (PD). Both respond to dopaminergic medications. Whether these disorders share a common pathophysiology is unclear. Because PD is associated with a loss of olfactory function, we compared the olfactory function, we compared the olfactory function of patients with RLS with control and PD patients. Using the University of Pennsylvania Smell Identification Test (UPSIT), olfactory function was found to be normal in patients with idiopathic RLS and significantly reduced in patients with PD. This suggests that the pathophysiology of RLS differs from PD, and that RLS likely is not a “forme fruste” or a preclinical sign of PD.