Neuropsychological functioning of 30 siblings of children with autism (AU-S), 28 siblings of children with mental retardation of (MR-S), and 30 siblings of children with developmental language delay (DLD-S) was compared. Two siblings, both AU-S, received diagnoses of pervasive developmental disorder (PDD). More siblings with cognitive disabilities were found in DLD-S than in AU-S. However, these differences disappeared after excluding diagnosed siblings or after accounting for family membership. In sum, despite the elevated incidence of PDD among AU-S, the neuropsychological functioning of the remaining siblings did not convey specific characteristics related to the genetic risk associated with autism, in contrast to the cognitive functioning of the DLD-S, which did reflect a genetic risk.