Only a limited topiramate dosing range (5-9 mg/kg/day) is approved by the U.S. Food and Drug Administration (FDA). We reviewed our topiramate dosing (mg/kg/d) and corresponding serum levels (microg/mL) (n = 77) in 41 children who were treated to clinical response or tolerability. The patients were divided into older (6-12 years [n = 21]) and younger (< or = 5 years [n = 20]) groups. Topiramate was given as monotherapy (n = 9), with an enzyme-inducing antiepileptic drug (n = 16) (phebarbital, phenytoin, or carbamazepine), or as polytherapy (n =17) (another antiepileptic drug). In the older children, there was a good dosage to serum level correspondence. However, younger children on topiramate monotherapy or cotherapy with an enzyme-inducing antiepileptic drug had relatively lower serum levels, but the serum level was increased if they were on polytherapy without an enzyme-inducing drug. This study supports a wider dosing range (7-22 mg/kg/day) of topiramate and dosage escalation beyond the approved range. Serum levels are useful in guiding topiramate dosing, especially in young children.