Two potentially contrasting hypotheses can be generated about sibling interactions involving a child with Down's syndrome or autism. Research on siblings would predict that learning disabled children adopt responsive roles. Studies of children with autism would predict impoverished interactions. Home observations were conducted on 30 sibling pairs involving children with autism or Down's syndrome. Both hypotheses were partially supported. All learning disabled children engaged in frequent bouts of interaction, usually directed by their sibling. While children with autism engaged in fewer bouts and imitated less, they did reciprocate their siblings' initiations. Sibling encounters provide a unique opportunity for such children to learn about social relationships.