RATIONALE: Fluoxetine improves affect in clinical syndromes such as depression and premenstrual dysphoric disorder. Little is known about fluoxetine's influence on mood changes after quitting smoking, which often resemble sub-clinical depression. OBJECTIVES: The present study, a re-analysis of previously published data (Niaura et al. 2002), examined fluoxetine's effect on changes in negative and positive affect following quitting smoking. METHODS: Adult smokers (n=175) without clinically significant depression were randomized on a double-blind basis to receive fluoxetine hydrochloride (30 or 60 mg daily) or placebo for 10 weeks in combination with cognitive-behavioral therapy (CBT) for smoking cessation. We postulated that fluoxetine would beneficially influence post-cessation changes in positive and negative affect. RESULTS: Mood change across treatment was analyzed using mixed linear modeling controlling for initial level of nicotine dependence, plasma fluoxetine metabolites, and change in cotinine (a nicotine metabolite) at each visit. Relative to placebo, those on 60 mg fluoxetine experienced an elevation in positive affect that increased across time [t(526)=2.50, P=0.01], and a reduction in negative affect that returned to baseline across time [t(524)=2.26, P=0.02]. There were no differences between 30 mg and placebo on changes in positive or negative affect. CONCLUSIONS: Results indicate that 60 mg of fluoxetine improves both positive and negative mood states after quitting smoking and that diminished positive affect may be an overlooked affective response to smoking cessation.