This study is a 26 week, randomized, parallel group, double blind comparison of PF-02545920 5 mg, PF-02545920 20 mg, and placebo dosed BID in the treatment of motor impairment of subjects with Huntington's Disease. A total of approximately 260 subjects are planned to be randomized in the study. Primary endpoint is the change from baseline in the Total Motor Score (TMS) assessment of the Unified Huntington Disease Rating Scale (UHDRS) after 26 weeks of treatment. secondary endpoints will include change from baseline in the Total Maximum Chorea (TMC) score of the UHDRS after 13 and 26 weeks of treatment and Clinical Global Impression-Improvement score after 13 and 26 weeks of treatment.