Background: The association between epilepsy and autism is consistently reported, with a wide range of prevalence rates. This may be attributed to the heterogeneity of the samples with respect to age, comorbidity, sex, and intellectual disability (ID). We aimed to compare the prevalence of epilepsy 1) among autistic patients with ID versus autistic patients without ID and 2) among male versus female autistic patients., Methods: We reviewed all data available from published reports (1963-2006) on autism and epilepsy and conducted a meta-analysis of 10 and 14 studies, respectively, to assess the relative risk (RR) of epilepsy in autism according to ID and gender. The pooled groups included 2112 (627 with IQ > or = 70, 1485 with IQ < 70) and 1530 (1191 male, 339 female) patients, respectively., Results: There was a strong discrepancy in relative risk (RR) according to IQ, with more autistic patients with ID having epilepsy (RR = .555; 95% confidence interval [CI]: .42-.73; p < .001). The pooled prevalence of epilepsy was 21.5% in autistic subjects with ID versus 8% in autistic subjects without ID. There was a strong discrepancy in RR according to sex, favoring comorbidity of epilepsy in autistic girls (RR = .549; 95% CI: .45-.66; p < .001). The male:female ratio of autism comorbid with epilepsy was close to 2:1 whereas the male:female ratio of autism without epilepsy was 3.5:1., Conclusions: The results of this meta-analysis indicate that risk for epilepsy in autism is a function of ID severity and distinguishes autism associated with epilepsy as a subgroup of autism by its male-female ratio.