Objectives : The aim of this study was to estimate the prevalences of electroencephalographic (EEG) abnormalities and epilepsy in children and adolescents with autism spectrum disorder (ASD). In addition, we intended to identify demographic and clinical correlates of epilepsy in ASD. Methods : A total of 140 children and adolescents (age $7.3{pm}4.8yrs$, 106 boys) with ASD underwent EEG from January 2010 to December 2013 at Asan Medical Center. Medical records were reviewed for demographic information, clinical characteristics, psychiatric diagnoses and comorbidities, EEG findings and neurological diagnoses. Results : The prevalences of EEG abnormalities and epilepsy in children and adolescents with ASD was 62.1% and 38.6%, respectively. In subjects with seizure-like movements, EEG abnormalities and epilepsy were more frequent than those without seizure-like movements (EEG abnormalities : 92.5% vs. 43.7%, p<.001 ; epilepsy : 90.6% vs. 5.7%, p<.001). ASD subjects who had epilepsy were older (p=.001), had lower full scale intelligence quotient (p<.001) and took more antipsychotics (p=.006) than those who did not. Conclusion : The prevalences of EEG abnormalities and epilepsy in our sample were similar to those from Western countries. Our results suggested a possible association of older age, lower intelligence quotient, and antipsychotics use with epilepsy in ASD. Conduct of further prospective study in a larger sample is needed.