A comparative observational study was undertaken of the spontaneous social interactions of children with high-functioning autism and Asperger's disorder. The sample comprised 20 children with high-functioning autism, 19 children with Asperger's disorder and 17 typically developing children matched on chronological age and overall mental age. A one-zero time sampling technique was used in live coding of the children's spontaneous social and play behaviours in the schoolyard. Few differences were found between children with high-functioning autism and Asperger's disorder on the dimensions of social interaction investigated. In contrast, the social behaviour of both clinical samples often deviated markedly from that of the typically developing children. The findings confirmed that although children with high-functioning autism or Asperger's disorder are often socially isolated relative to their typically developing peers, they are capable of spontaneously engaging socially with other children. The results were supportive of the hypothesis that Asperger's disorder is on a continuum with autistic disorder. (Contains 1 note, 6 tables and 1 figure.)