BACKGROUND: The purpose of this study was to assess the efficacy and safety of entacapone, a catechol-O-methyltransferase (COMT) inhibitor in Parkinson's disease (PD) patients with wearing-off phenomenon. METHODS: A total of 197 PD patients were included in this 2-month multi-center, randomized, placebo-controlled, double blind, parallel-group study. After a 2-week screening period, each patient was randomly allocated to receive either entacapone (n=98) or placebo (n=99) as an adjunct to levodopa. The efficacy was evaluated with the changes of "on" and "off" time percentage while awake, the reduction of the levodopa dose, Unified Parkinson's Disease Rating Scale (UPDRS), and the clinical global impression (CGI) by the examiner. RESULTS: The percentage of "on" time increased by 9.4 +/- 18.0% in the entacapone group, 7.4 +/- 15.6% in the placebo group. The percentage of "off" time was reduced by 8.6 +/- 16.9% in the entacapone group, 6.6 +/- 18.2% in the placebo group. These parameters did not show a statistical significance between the two groups. However, the levodopa dose was significantly reduced in the entacapone group (51.6 +/- 154.5 mg/day) compared with the placebo group (0.7 +/- 130.0 mg/day) (p=0.009). The total and motor scores of the UPDRS were significantly decreased in the entacapone group (p=0.039, p=0.017, respectively). The most common adverse drug reactions in the entacapone group were urine discoloration (22%), dyskinesia (13%), dizziness (7%). CONCLUSIONS: Entacapone was a safe and well-tolerated drug. Although the changes of "on" and "off" time were not significant, entacapone showed an overall significant beneficial effect in the PD patients with wearing-off phenomenon.