Angelman syndrome (AS) is a genetic disorder with characteristic clinical and EEG findings. We report here the results of long-term follow-up studies on the epileptic seizures and EEG findings of 23 cases of deletion type AS confirmed by FISH analysis, including seven cases previously reported by Matsumoto et al. in 1992. The age at last follow-up in 23 patients was from 1 to 37 years of age (average: 18.0 years), with 10 patients (43.5%) in their 20s, and five over 30. Epileptic seizures were seen in all patients, and the age at seizure onset ranged from 3 to 50 months (average: 21.7 months). Status epilepticus was seen in 11 patients (47.8%). The percentages of cases seizure-free for more than 3 years were 25% (4/16) at 10 years of age, 70% (7/10) at 20, and 80% (4/5) at 30. The EEG findings were classified into six patterns according to the previous report: N (no spike, including focal slow waves), HVS (diffuse high-voltage slow bursts with or without spikes), F (focal spikes or multifocal spikes), S (diffuse spike and waves), C (continuous diffuse spike and waves), Hy (hypsarrythmia or hypsarrhythmia like waves). Hy was noted at ages 0-2 years in two cases. C was observed from the ages 2 to 15 years, being most frequently noted at 3-6 years of age, and it was never seen after 16 years of age. S was observed from ages 1 to 21 years. F was seen from 2 to 21 years of age, and most frequently during the ages of 2-7 years. HVS was seen from 0 years, and still remained after the age of 20. After 22 years of age, all patients showed N pattern including focal slow waves. One of the two patients who had bilateral frontal dominant delta slow waves in their 30s, had a recent seizure. Even if the spikes disappear with age, when bi-frontal focal slow waves remain, seizures may occur even in patients over 30.