OBJECTIVE: To determine what clinical symptoms clinicians have been using to distinguish between Asperger's disorder (AsD) and autistic disorder (AD). METHOD: Parents of children and adolescents with high-functioning AD (n = 48) and AsD (n = 69) were given a structured interview based on DSM-III-R and ICD-10 diagnostic criteria. Information regarding early and current symptom presentation and family, developmental, and verbal mental age information were collected. Logistic regression analyses were conducted to determine which variables best predicted clinician's diagnosis. RESULTS: A number of clinical variables predicted diagnosis. Delayed language onset was the only variable of the family and developmental variables that predicted diagnosis. The AsD group was also significantly higher than the AD group in verbal mental age. CONCLUSION: Clinicians appear to be diagnosing AsD and AD on the basis of published research and case study accounts. The findings question whether DSM-IV and ICD-10 criteria adequately describe the AsD individual, particularly in the communication domain.