In this open-label study, the safety, tolerability, and pharmacokinetics of oxcarbazepine as monotherapy or adjunctive therapy were studied in infants and young children with partial seizures. In a 30-day treatment phase, oxcarbazepine was titrated from 10 mg/kg/day to 60 mg/kg/day. Blood samples for analysis of the oxcarbazepine metabolite, the 10-monohydroxy derivative (MHD), were obtained at regular intervals. Patients completing the treatment phase entered a 6-month extension phase. Safety and tolerability were assessed throughout the study. Twenty-four patients (mean [range] age, 20.4 [2-45] months) were enrolled. Nineteen (79%) patients completed the treatment phase and, together with one patient who discontinued prematurely during the treatment phase, entered the extension phase. Thirteen of 20 (65%) patients completed the extension phase. The most common adverse events were pyrexia, ear infection, and irritability. Whether patients (n = 23) received enzyme-inducing antiepileptic drugs or not, MHD concentrations were consistent with those predicted from a linear, one-compartment, population-pharmacokinetic model based on a model previously fitted for 3- to 17-year-old children. Oxcarbazepine was safe and well tolerated in infants and young children. The pharmacokinetic profile of MHD was predicted by extension of a model based on older children.