Objective: This randomized, double-blind, placebo-controlled study assessed the efficacy and tolerability of several modafinil dosing regimens in children with attention-deficit/hyperactivity disorder (ADHD) to determine whether modafinil can be given once daily in pediatric ADHD. Method: Children and adolescents (age range, 6-13 years) (N = 248) with DSM-IV-defined ADHD were enrolled in a 4-week, double-blind, placebo-controlled study, conducted February-May 2002. The group was assigned to receive oral (100-mg tablets) modafinil 300 mg once daily (300 mg in the morning followed by placebo at midday), modafinil 300 mg as a divided dose (100/200 mg or 200/100 mg), or matching placebo. In children weighing ≥ 30 kg, a higher dose of 400 mg (200/200 mg) was evaluated. Efficacy measures included the teacher-rated School Version and clinician-rated Home Version of the ADHD Rating Scale-IV and the parent-completed Conners' ADHD/DSM-IV Scales. Results: 223 children completed the study. Those who received modafinil 300 mg once daily showed a significantly greater improvement (change from baseline) than those who received placebo in symptoms of ADHD across all rating scales and subscales (all p < .05). Divided 300-mg doses of modafinil provided some significant but inconsistent improvements in ADHD symptoms. In children weighing ≥ 30 kg, modafinil 400 mg (200/200 mg) was significantly superior to placebo on clinician- and parent-completed scales (all p < .05). Insomnia was the only adverse event to occur with significantly greater frequency in a modafinil group (200/100) than in the placebo group (14% vs. 2%) (p = .03). Conclusion: Modafinil significantly improved ADHD symptoms in children. Once-daily dosing (300 mg) provided the most consistent improvement in symptoms. All dosing regimens of modafinil were well tolerated.