This paper describes a total population study of Asperger syndrome using a two-stage procedure. All school children in an outer Göteborg borough were screened. Final case selection based on clinical work-up showed a minimum prevalence of 3.6 per 1.000 children (7-16 years of age) using Gillberg and Gillberg's criteria and a male to female ratio of 4:1. Including suspected and possible Asperger syndrome cases, the prevalence rose to 7.1 per 1.000 children and the male:female ratio dropped to 2.3:1. These findings are discussed as they relate to previously published results in the field and to findings obtained using Szatmari et al.'s and ICD-10 draft criteria for the disorder.

THE ability to attribute mental states to others ('theory of mind') pervades normal social interaction and is impaired in autistic individuals. In a previous positron emission tomography scan study of normal volunteers, performing a 'theory of mind' task was associated with activity in left medial prefrontal cortex. We used the same paradigm in five patients with Asperger syndrome, a mild variant of autism with normal intellectual functioning. No task-related activity was found in this region, but normal activity was observed in immediately adjacent areas. This result suggests that a highly circumscribed region of left medial prefrontal cortex is a crucial component of the brain system that underlies the normal understanding of other minds.

The high-functioning Autism Spectrum Screening Questionnaire (ASSQ) is a 27-item checklist for completion by lay informants when assessing symptoms characteristic of Asperger syndrome and other high-functioning autism spectrum disorders in children and adolescents with normal intelligence or mild mental retardation. Data for parent and teacher ratings in a clinical sample are presented along with various measures of reliability and validity. Optimal cutoff scores were estimated, using Receiver Operating Characteristic analysis. Findings indicate that the ASSQ is a useful brief screening device for the identification of autism spectrum disorders in clinical settings.

The Wechsler Intelligence Scale for Children-Revised (WISC-R) was applied (in a Swedish version) in 120 children with Asperger syndrome, autistic disorder, and attention disorders. Using stepwise logistic regression analysis, the WISC's discriminating ability was investigated. The overall rate of correct diagnostic classification was 63%. Further, WISC profiles were analysed within each group. The group with autistic disorder was characterised by a peak on Block Design. The Asperger syndrome group had good verbal ability and troughs on Object Assembly and Coding. The group with attention disorders had troughs on Coding and Arithmetic. The results suggest that Kaufman's Verbal Comprehension, Perceptual Organisation and Freedom from Distractibility factors rather than verbal IQ and performance IQ account for the variance on the WISC. Furthermore, the Asperger syndrome and autistic disorder groups differed in respect of "fluid" and "crystallised" cognitive ability.

Twenty-eight children referred with a preliminary diagnosis of autistic disorder under age 3 yrs were extensively examined from the neuropsychiatric point of view and followed up for several months to several years. A diagnosis of autistic disorder was confirmed in 75% of the cases. A variety of associated medical conditions was identified. It was concluded that autism can be diagnosed in a substantial proportion of cases before age 3 yrs and that the neurobiological background is similar to that seen in older autistic children.