Guanfacine, an alpha(2A)-adrenoceptor agonist, is available in the US as an extended-release (ER) tablet for the treatment of attention deficit hyperactivity disorder (ADHD) in children and adolescents (aged 6-17 years). In two large, randomized, double-blind, placebo-controlled trials of 8 and 9 weeks' duration, guanfacine ER (1-4 mg once daily) was effective in reducing the symptoms of ADHD (hyperactivity, impulsivity and inattention) in children and adolescents. There were significant reductions (denoting improvements) from baseline in ADHD rating scale IV (ADHD-RS-IV) total scores (the primary endpoint), compared with placebo. Oppositional symptoms were also significantly reduced from baseline in children with ADHD with oppositional symptoms who received guanfacine ER in a randomized, double-blind, placebo-controlled trial. Improvements in ADHD symptoms were sustained over 24 months in two noncomparative, open-label extension trials in children and adolescents who received guanfacine ER at an optimized dosage of 1-4 mg/day. Guanfacine ER was relatively well tolerated in clinical trials in children and adolescents. The most common treatment-emergent adverse events were somnolence-related, and tended to resolve over time.