Zonisamide is a new type of benzisoxazole derivative, first marketed in Japan in 1989. This study analyzed: (1) the drug's efficacy by seizure and epilepsy type in a total of 1008 patients treated during the development of zonisamide in Japan; (2) the effectiveness of zonisamide for 726 newly-diagnosed patients treated with zonisamide postmarketing; and (3) 50 patients with generalized epilepsies and epileptic syndromes (idiopathic generalized epilepsies, symptomatic generalized epilepsies, Lennox-Gastaut syndrome, Doose syndrome, and West syndrome), and 19 patients with undetermined epilepsies and specific syndromes (refractory grand mal in childhood, severe myoclonic epilepsy in infancy, other undetermined epilepsy, familial essential myoclonic epilepsy, and mitochondrial encephalomyopathy with ragged-red fibers). Analysis of study results showed that among all patients treated, zonisamide was highly effective for the treatment of idiopathic generalized epilepsy, temporal lobe epilepsy, and other partial epilepsies. The compound was also effective for other symptomatic generalized epilepsies. In 50 patients with generalized epilepsies, and 19 with undetermined epilepsies and specific syndromes, seizure frequency was reduced by >50% with monotherapy or two-drug therapy with zonisamide. Zonisamide was effective not only for partial epilepsies and generalized epilepsies but also for undetermined epilepsies and specific syndromes such as myoclonus epilepsy.