OBJECTIVE: The authors assessed the efficacy of once-daily atomoxetine administration in the treatment of children and adolescents with attention deficit hyperactivity disorder (ADHD). METHOD: In a double-blind study, children and adolescents with ADHD (N=171, age range=6-16 years) were randomly assigned to receive 6 weeks of treatment with either atomoxetine (administered once daily) or placebo. RESULTS: Outcomes among atomoxetine-treated patients were superior to those of the placebo treatment group as assessed by investigator, parent, and teacher ratings. The treatment effect size (0.71) was similar to those observed in previous atomoxetine studies that used twice-daily dosing. Parent diary ratings suggested that drug-specific effects were sustained late in the day. Discontinuations due to adverse events were low (less than 3%) for both treatment groups, and no serious safety concerns were observed. CONCLUSIONS: Once-daily administration of atomoxetine is an effective treatment for children and adolescents with ADHD.