PURPOSE: To determine whether sustained-release bupropion promotes smoking reduction leading to smoking cessation among persons who wish to reduce their amount of smoking, but who are unwilling to quit or who perceive themselves as being unable to quit. METHODS: Current smokers were assigned randomly to receive either sustained-release bupropion (150 mg twice daily) or matching placebo. During an initial 6-month smoking reduction phase, those who were willing to quit entered a 7-week cessation phase, during which study medication was continued. RESULTS: Four-week continuous abstinence rates were 14% (41/295) in the bupropion group and 8% (25/299) in the placebo group (P = 0.02) during treatment. However, this benefit did not continue after treatment was stopped; subsequent continuous abstinence rates were 7% (20/295) in the bupropion group and 5% (16/299) in the placebo group (P = 0.50). Similar proportions of subjects entered the cessation phase in both treatment groups (38% [n = 113] of those in the bupropion group and 34% [n = 101] of those in the placebo group), although the time until a cessation attempt was shorter for those taking bupropion (median, 64 days vs. 118 days, P = 0.008). The extent of smoking reduction (measured by urinary cotinine concentrations) among the 327 subjects who did not enter the cessation phase was significantly greater (P <0.05) in those treated with bupropion during the reduction treatment phase, but not during the month 12 follow-up visit (P = 0.25). CONCLUSION: Sustained-release bupropion, when used in smokers initially not willing to make a cessation attempt, can help sustain smoking reduction while subjects are on active medication, reduce the time until the next cessation attempt, and increase short-term abstinence rates. However, these benefits were modest and not sustained after bupropion was discontinued.