We determined the efficacy of and tolerability to zonisamide (ZNS) in newly diagnosed patients with infantile spasms. ZNS, 4-20 mg/kg per day, was introduced as an add-on therapy or monotherapy in 27 children with infantile spasms (cryptogenic, 2; symptomatic, 25). The dosage was initially 2-4 mg/kg per day, and then was increased by 2-5 mg/kg every 2-4 days until the seizures disappeared. Nine (33.3%) out of the 27 patients who were administered ZNS exhibited the disappearance of seizures. ZNS was effective in all the cryptogenic cases and seven (28.0%) of the symptomatic cases. The effective daily doses were 5-12.5 mg/kg (mean, 7.8 mg/kg), and the daily dosages were 40-100 mg (mean, 61.1 mg). The steady-state plasma ZNS concentrations were almost within the therapeutic range. The mean time interval between the start of ZNS therapy and the seizure disappearance was 5.0 days. Six (75.0%) of eight effective cases, the exception being one for whom EEG was not performed after the therapy, showed the disappearance of hypsarrhythmia. The recurrence of seizures were observed in four of the nine cases. No adverse reaction to ZNS was noted in any patient. In conclusion, ZNS treatment is considered to be worthwhile trying, for early stage therapy for infantile spasms.