Parkinson's disease (PD) is a neurodegenerative disorder characterized by a variety of motor symptoms including freezing of gait (FOG), in which walking is transiently halted as if the patient's feet were 'glued to the ground'. Treatment of FOG is still challenging. Although L-threo-3,4-dihydroxyphenylserine (L-DOPS), a precursor of noradrenaline, has been on the market in Japan because of its beneficial effect for FOG, clinical use of L-DOPS has been far from satisfying. However, the fact that there were some responders to L-DOPS encouraged us to hypothesize that the enhancement of L-DOPS concentration in the brain by the co-administration of L-DOPS and a catechol-O-methyl transferase (COMT) inhibitor, which is expected to interrupt L-DOPS metabolism in the peripheral circulation, would be beneficial for FOG. Based on our hypothesis, we conducted a preliminary study with a small number of participants with FOG. Of the 16 PD patients with FOG who completed this study, group 1 (n=6) received L-DOPS co-administered with entacapone, which is a COMT inhibitor used worldwide as an anti-parkinson drug, group 2 (n=5) received entacapone alone, and group 3 (n=5) received L-DOPS alone. Only the patients in group 1 showed a significant improvement in FOG. Moreover, the beneficial effect was observed only in patients with levodopa-resistant FOG. This result supports our hypothesis, at least in patients with levodopa-resistant FOG, and shows that the co-administration of L-DOPS and entacapone could be a new strategy for FOG treatment.