Depression is the most common psychiatric complication in patients with Parkinson's disease (PD). Istradefylline, a new anti-parkinsonian agent with completely different mechanism, improves depression-like symptoms in an experimental disease model; however, there is no report of its effects in PD patients. In this study, the effectiveness of istradefylline for treatment of mood disorders in patients with PD was examined in an open-label trial. Thirty PD patients were enrolled. All patients had scores of higher than cut-off level in at least one of the following batteries: Snaith-Hamilton Pleasure Scale Japanese version (SHAPS-J), Apathy scale, or Beck Depression Inventory-2nd edition (BDI). Following study enrollment, all patients received 20 mg of istradefylline, and the dose was increased to 40 mg after 4 weeks. Results from these 3 batteries and the Unified Parkinson's Disease Rating Scale (UPDRS) score were assessed every 2-4 weeks until 12 weeks and the changes in these scores were analyzed. Following administration of istradefylline, the scores of SHAPS-J, Apathy scale, and BDI were significantly improved over time. Significant improvement was also found in the UPDRS score; however, no significant correlation was observed between the score change in these 3 batteries and UPDRS motor function. This is the first study to show the effectiveness of istradefylline for treatment of mood disorders in PD independent of improvement of parkinsonian motor symptoms. In the future, this should be confirmed in a double-blind placebo-controlled trial.