Objeictivc: A multicenter open-label clinical trial was conducted to evaluate the clinical usefulness of topiramate (TPM) as primary or adjunctive therapy for infantile spasms in the postmarketing period in China. Methods: Thirty-four centers participated in the trial. Patients included in the study had 1 or more seizures per day before treatment. One hundred twenty (22.1%) very young patients with an age younger than 6 month and 64.2% of patients were younger than 1 year at start of treatment. All patients received a starting dose of 0.5 to 1 mg kg(-1) d(-1) TPM twice daily. The dosage was increased by 0.5 to 1 mg kg(-1) d(-1) every 5 to 7 days up to 3 to 5 mg kg(-1) d(-1). The resulting range of the total TPM dosage was 25 to 200 mgd(-1) (3-57-20 mg kg(-1) d(-1)), with a median value of 73.9 mg d(-1). Seizure outcomes were measured by intention-to-treat analysis. Patients were seen by a neurologist, and their data were evaluated at the day of inclusion and after 4, 8, 12, 16, and 20 weeks (from visit 1 to visit 5) of treatment. Results: Five hundred forty-four patients entered the study. After 20 weeks of TPM treatment, 239 patients (43.9%) were seizure-free. A higher proportion of patients in the monotherapy group than in the add-on therapy group showed a seizure rate reduction. An increase in seizure frequency was observed in 8 patients (1.5%) during the 20-week treatment period. Nineteen patients were withdrawn before completing the study, and in 46 cases, some data of the structured data files and questionnaires were missing. No efficacy of TPM treatment was recorded in these cases. Adverse effects occurred in 211 patients (38.8%). Most frequent side effects were anorexia and somnolence. Conclusions: Topiramate proved to be an effective and safe monotherapy and add-on therapy in patients with infantile spasms younger than 1 year.