Background: Children with autistic spectrum disorders (ASD) and mild to borderline intellectual disability (ID) have less adaptive behaviour and more behaviour problems than children with mild to borderline ID. Social information processing appears to be an important mechanism in the explanation of the socially inadequate behaviour of children with mild to borderline ID; however, little is known about the social information processing skills of children with ASD and mild to borderline ID. Method: In the present study, a total of 136 boys in the age of 10-14 years participated; 26 with ASD (specifically Pervasive Developmental Disorder-Not Otherwise Specified) and mild to borderline ID, 54 with mild to borderline ID without ASD and 56 typically developing boys. They completed the "Social Problem Solving Test" to measure their social information processing. Results: The research results show boys with PDD-NOS and mild to borderline ID to differ from typically developing boys in their encoding of information; they focus on negative and emotional information in the social situation. They differ from boys with mild to borderline ID in response generation, evaluation of inadequate solutions (aggressive and submissive responses) and assertive response decision. Conclusions: The present study extends our knowledge regarding social information processing of children with ASD (PDD-NOS) and mild to borderline ID. This knowledge may be helpful in designing and adapting programmes (e.g. social skills training, self-management training) for the management of behaviour problems and development of adaptive behaviour of children with ASD and mild to borderline ID.