Objective: Sibling recurrence risk in autism has been estimated to be approximately 10%. This study investigated sub-syndromal autistic impairments among siblings of probands with pervasive developmental disorders. Method: The authors used the Social Responsiveness Scale to obtain quantitative assessments of autistic social impairment in three groups of proband-sibling pairs: 1) autistic children from multiple-incidence families and their closest in age nonautistic brothers (N=49 pairs); 2) children with any pervasive developmental disorder, including autism, and their closest-in-age brothers (N=100 pairs), and 3) children with psychopathology unrelated to autism and their closest-in-age brothers (N=45 pairs). Results: Sibling Social Responsiveness Scale scores were continuously distributed and substantially elevated for both the autistic and pervasive developmental disorder groups. Highest scores (i.e., greatest impairment) were seen among siblings of autistic probands from multiple-incidence families, followed by siblings of probands with any pervasive developmental disorder, then siblings of probands with psychopathology unrelated to autism. Conclusions: Taken together with previous findings, these results support the notion that genetic susceptibility factors responsible for common, subsyndromal social impairments may be related to the causes of categorically defined pervasive developmental disorders.