Two primary care practices were used to recruit adults with and without disability. Disability groups included autism, Down syndrome, cerebral palsy, and mental retardation. The patients without disability had an epilepsy prevalence rate of 1%. The prevalence of epilepsy within the disability groups was 13% for cerebral palsy, 13.6% for Down syndrome; 25.4% for autism, 25.5% for mental retardation, and 40% for adults with both cerebral palsy and mental retardation. During the decades of adulthood, the prevalence of epilepsy declined for those with cerebral palsy and mental retardation. The prevalence of epilepsy increased with advancing years for adults with Down syndrome, autism, and those without disability. Nonetheless, during each decade the prevalence of epilepsy was higher in all of the disability groups compared to those without disability.