Autism is a genetically heterogeneous entity. Although numerous studies have demonstrated a strong genetic basis, no clear etiology has been identified to date. Recently, two studies have demonstrated an increased risk of autism in children born to fathers over the age of 40. However, given the large male-to-female predominance of autism, it is likely that new mutations on the X chromosome account for a significant number of affected cases. Due to the maternal origin of the X chromosome in males, we hypothesize that advanced maternal-grandpaternal age may also be a risk factor for autism. Precedence for this theory exists with other X-linked disorders such as Duchenne muscular dystrophy and Rett syndrome. Additionally, it has been demonstrated that maternal psychiatric illness, but not paternal psychiatric illness, is more prevalent among parents of children with autism. Using anonymous surveys of families with autistic children, we seek to identify the ages of grandparents at the time the parents were born in order to determine if advanced maternal-grandpaternal age is associated with an increased risk for autism when adjusted for advanced maternal and paternal age. Additionally, we will seek out sister-pairs in order to identify any statistical significance between the ages of the maternal grandfather at delivery of each sister. If advanced maternal-grandpaternal age is, in fact, a risk factor, it would help direct molecular researchers towards genes on the X chromosome as potential etiologies for autism. Also, further study of potential mutagenic exposures in the environment of grandparents may help elucidate the reason for the increasing incidence of autism in recent decades.