The frequency and clinical presentation of autism in 28 probands with tuberous sclerosis complex (TSC) are reported and risk factors that may influence the development of autism in TSC are examined. Eight probands meet ICD-10 and DSM-IV criteria for autism, an additional 4 meet criteria for pervasive developmental disorder (PDD). Twelve TSC probands with autism/PDD are compared to 16 TSC probands without these conditions for factors which may underlie the association of autism and TSC. A specific seizure type, infantile spasms, as well as mental retardation, are increased in the TSC, autistic/PDD group. Furthermore, rates of social phobia and substance abuse are elevated among first-degree relatives of TSC probands with autism compared to first-degree relatives of TSC probands without autism. Implications of these findings in understanding the association of autism and TSC are discussed.