Although restless legs syndrome (RLS) commonly accompanies Parkinson disease (PD), the mechanism of RLS development in PD is still unclear. We investigated the prevalence of RLS in Korean patients with PD, and the possible contributing factors to the development of RLS in those patients. Four hundred forty-seven consecutive patients with PD were interviewed and examined. Among them, 73 patients (16.3%) were diagnosed with RLS. PD patients with RLS had a longer duration of PD symptoms, more severe PD disability, a greater degree of cognitive decline, and a longer duration of antiparkinson therapy than those without RLS. Multivariate logistic regression analysis revealed that the duration of antiparkinson therapy was the most significant factor contributing to the development of RLS in patients with PD. The present results support a higher prevalence of RLS in patients with PD and suggest that long-term antiparkinson therapy, rather than PD itself, may contribute to the development of RLS.