Evaluate safety and efficacy of mixed amphetamine salts extended release (MAS XR) in adults with attention-deficit/hyperactivity disorder (ADHD). 10-week interim analysis of the Quality of life, Effectiveness, Safety, and Tolerability (QU.E.S.T.) trial, an ongoing, 30-week, open-label, multicenter investigation of once-daily MAS XR (10-60 mg/day) in adults (>or=18 years of age) with ADHD in community practice settings. With up to 10 weeks of open-label MAS XR 10 to 60 mg/day (final visit mean dose: 37.2 mg/day), 725 adults exhibited rapid, sustained improvement in ADHD symptoms. At end point, significant decreases from baseline were seen in ADHD Rating Scale IV total scores (-19.8+/-11.6; P<.0001), hyperactivity/impulsivity subscale (-8.1+/-6.1; P<.0001), and inattentive subscale (-11.6+/-6.7; P<.0001). Most subjects (74.4%) were rated as very much/much improved. Based on the 36-item Short Form Health Survey (version 2), significant improvements in quality of life were seen in the domains of general health, physical and mental health, vitality, and social, emotional and physical role functioning (P<.0001). Few subjects (6.9%) withdrew due to adverse events; the most common MAS XR-related adverse events were decreased appetite and dry mouth (19.2% each), insomnia (17.8%), and headache (16.8%). In adults with ADHD, MAS XR treatment is generally safe and demonstrates significant improvement in ADHD symptoms and related quality of life.