The primary purpose of this study is a comparison of abstinence maintenance in subjects randomized to double-blind varenicline or placebo from Weeks 13 -24 in subjects who responded to an intial 12-week open label course of varenicline, with post-treatment follow-up of smoking status to Week 52.

Inclusion Criteria:

• Subjects must have smoked an average of at least 10 cigarettes per day during the past year and over the month prior to the screening visit, with no period of abstinence greater than 3 months in the past year.

Exclusion Criteria:

• Subjects with clinically significant cardiovascular disease in the past 6 months.