In a previously described population of 314 autistic and 237 dysphasic nonautistic children, after exclusion of 12 autistic girls with Rett syndrome, 14% (42 of 302) of autistic children and 8% (19 of 237) of dysphasic children had epilepsy (P = .03). The major risk factors for epilepsy were severe mental deficiency and the combination of severe mental deficiency with a motor deficit. In autistic children without severe mental deficiency, motor deficit, associated perinatal or medical disorder, or a positive family history of epilepsy, epilepsy occurred in 6% (10 of 160) which was analogous to the 8% (14 of 168) found in similar dysphasic nonautistic children. The language subtype of verbal auditory agnosia is associated with the highest risk of epilepsy in autistic (41%, 7 of 17) and dysphasic (58%, 7 of 12) children. The higher percentage of epilepsy in autistic girls, 24% (18 of 74) compared with boys 11% (25 of 228) (P = .003), is attributed to the increased prevalence of cognitive and motor deficit in girls. Once the risk attributable to associated cognitive and motor disabilities is taken into account, there is no difference in the risk of epilepsy between autistic and nonautistic dysphasic children.