There is broad agreement that genetic influences are central in the development of idiopathic autism. Whether relatives manifest genetically related milder phenotypes, and if so how these relate to autism proper, has proved a more contentious issue. A review of the relevant studies indicates that relatives are sometimes affected by difficulties that appear conceptually related to autistic behaviors. These range in severity from pervasive developmental disorders to abnormalities in only one area of functioning, and possibly extend to related personality traits. Issues involved in clarifying the components of milder phenotypes and their relationship to autism are outlined.