BACKGROUND: Data remain limited on treatment strategies for adults with attention-deficit/hyperactivity disorder (ADHD). This study evaluated the efficacy and safety of an extended-release, once-daily formulation of bupropion (XL) in the treatment of adults with ADHD. METHODS: This multisite, placebo-controlled, 8-week prospective trial evaluated 162 adult patients diagnosed with ADHD (combined and inattentive types). Subjects were treated with up to 450 mg/day of bupropion XL. The primary efficacy endpoint was the proportion of ADHD responders (defined as at least a 30% reduction in the investigator-rated ADHD Rating Scale score) at week 8 (last observation carried forward [LOCF]). RESULTS: Bupropion XL responders (53%) exceeded placebo responders (31%) (p =.004 at week 8) with a significantly greater proportion of bupropion XL responders as early as week 2 (p = .01). Treatment effect size calculated for the ADHD Rating Scale total score was .6. Bupropion XL appeared to provide sustained benefit throughout the day compared with placebo (morning p =.033, afternoon p =.004, evening p = .024). Bupropion XL was safe and well tolerated, with no serious or unexpected adverse events and a low rate of drug-related study discontinuation (5%). CONCLUSIONS: The results from this multisite study indicate that bupropion XL is an effective and well-tolerated nonstimulant treatment for adult ADHD.