Although the reported incidence of epilepsy associated with trisomy 18 is 25–50%, there have been no detailed descriptions of the characteristics of trisomy 18-related epilepsy. We investigated the characteristics of epilepsy in children with trisomy 18 who remained alive for over 1 year by sending questionnaires to pediatric neurologists belonging to the Kyoto Multi-institutional Study Group of Pediatric Neurology. Eleven patients with trisomy 18 were enrolled (age at the study, from 15 to 134 months; median, 43 months), of whom seven (64%) had epilepsy. The age at seizure onset ranged from 1 to 42 months (median: 11 months). Among the seven patients with epilepsy, two had focal epilepsy, four had generalized epilepsy including infantile spasms in three, and the remaining one had an unclassified type. Seizure seminology included complex partial seizures in both the patients with focal epilepsy. At the time of the investigation, three children with generalized epilepsy still had daily seizures, while the remaining four were seizure-free. In conclusion, the characteristics of epilepsy in patients with trisomy 18 were as follows: over half of the children developed epilepsy during infancy or early childhood; infantile spasms might be one of the common epileptic syndromes; the epilepsy was intractable in half of the children, especially in those with generalized epilepsy. © 2013 Wiley Periodicals, Inc.