Purpose: To assess the efficacy and safety of levetiracetam (LEV) as adjunctive therapy in children with treatment-resistant partial-onset seizures. Methods: Children (aged 6-12 years) with treatment-resistant partial-onset seizures receiving one standard antiepileptic drug (AED) were eligible. After a 4-week baseline period, children received LEV in a 6-week titration phase (target dose, 40 mg/kg/day) followed by an 8-week evaluation phase. Seizure frequency during the evaluation period with individualized LEV doses (20-40 mg/kg/day) were compared with the 4-week baseline seizure frequency. Plasma concentrations of LEV and other AEDs were determined to evaluate potential drug interactions. Results: Twenty-four subjects enrolled and received LEV; 23 entered the evaluation phase, and 22 completed the evaluation phase. Compared with their baseline seizure frequency, 12 (52%) of 23 subjects entering the evaluation phase had their seizure frequency decrease by >50%. Two subjects remained seizure free during the entire evaluation period. LEV did not significantly affect plasma concentrations of any concomitant AED during this study, and no alteration of mean clinical laboratory values was observed. The most commonly reported adverse events were headache, infection, anorexia, and somnolence. Conclusions: This open-label study of adjunctive LEV therapy (at 20-40 mg/kg/day) suggests that LEV is effective, safe, and well tolerated in children ages 6-12 years with treatment- resistant partial-onset seizures. A randomized, placebo-controlled, double-blind trial of LEV adjunctive therapy in children with treatment-resistant partial-onset seizures is needed and ongoing to confirm these open-label findings.