The objective of the study was to compare the efficacy and tolerability of once-daily atomoxetine (≤1.8 mg/(kg day) with those of placebo in children and adolescents (aged 6–16 years) with attention-deficit/hyperactivity disorder [ADHD (DSM-IV)]. This randomized, placebo-controlled, double-blind trial was conducted in Russia. The primary efficacy measure was baseline-to-end point changes in Attention-Deficit/Hyperactivity Disorder Rating Scale-IV-Parent Version: Investigator-Administered and Scored (ADHDRS-IV-Parent:Inv) total score. Tolerability measures included treatment-emergent signs and symptoms (TESS), laboratory values and weight. Compared with patients in the placebo group (n = 33), patients treated with atomoxetine (n = 72) with a mean final dose of 1.4 mg/kg showed significantly greater improvement in ADHDRS-IV-Parent:Inv total score (least-squares mean: atomoxetine, –15.8; placebo, –11.4; p = 0.013). The most common TESS in the atomoxetine group included anorexia [atomoxetine, n = 13 (18.1%); placebo, n = 2 (6.1%)], somnolence, n = 11 versus n = 3 (15.3% vs. 9.1%, respectively), abdominal pain n = 9 versus n = 1 (12.5% vs. 3.0%, respectively) and nausea, n = 8 versus n = 1 (11.1% vs. 3.0%, respectively). Seven patients in the atomoxetine group and two in the placebo group experienced clinically important weight loss during the study (≥7% from baseline; mean change, kg: atomoxetine, –0.6; placebo, 0.1; p = 0.032). Atomoxetine is efficacious in improving ADHD symptoms in children and adolescents. Atomoxetine treatment may be associated with a numerically higher incidence of anorexia, somnolence, abdominal pain and nausea, as well as statistically greater losses in body weight.