Neurobiological and behavioural studies of possible left hemisphere dysfunction in autism have generated conflicting results. Left hemisphere dysfunction may manifest in autism only in tasks that invoke executive functions. Moreover, left hemisphere dysfunction may underpin autism but not Asperger's disorder. We thus aimed to systematically investigate reports of anomalous lateralization in individuals with high-functioning autism and Asperger's disorder. Two of the tasks were sensitive to executive dysfunction: a serial choice reaction-time task and a Posner-type paradigm; the remaining tasks instead investigated aspects of perceptual lateralisation. Compared with age- and IQ-matched controls, the autism group displayed deficiencies in right hemispace (and by implication, left hemisphere) performance on both executive function tasks; however, this group demonstrated normal lateralization effects on the nonexecutive, visual-perceptual tasks. In contrast, the Asperger's disorder group showed similar laterality effects to their age- and IQ-matched controls on both executive and nonexecutive function tasks. The etiological relevance of this neurobehavioral dissociation between high-functioning autism and Asperger's disorder was discussed; in particular, it was suggested that the period where dominance shifts from right to left hemisphere is important in whatever process might dictate the emergence of either autism or Asperger's disorder.