OBJECTIVE: Studies of families ascertained through a single autistic proband suggest that the genetic liability for autism may be expressed in nonautistic relatives in a phenotype that is milder but qualitatively similar to the defining features of autism. The objective of this study was to examine behaviors that may define this broader phenotype in relatives ascertained through two autistic siblings. METHOD: The authors used a semistructured family history interview to compare the rates of social and communication deficits and stereotyped behaviors in relatives ascertained through two autistic siblings (families with multiple-incidence autism; 25 families) with the rates in relatives of Down syndrome probands (30 families). RESULTS: Higher rates of social and communication deficits and stereotyped behaviors were found in the relatives in the families with multiple-incidence autism. CONCLUSIONS: These data suggest that further studies should be undertaken to delineate the boundaries of the broader autism phenotype and that this broader phenotype should be included in some future genetic analyses of this disorder.Abstract Teaser