Growing awareness of Autism Spectrum Disorders (ASD) has increased the demand for diagnostic services in adulthood. High rates of mental health problems have been reported in young people and adults with ASD. However sampling and methodological issues mean prevalence estimates and conclusions about specificity in psychiatric co-morbidity in ASD remain unclear. A retrospective case review of 859 adults referred for assessment of ASD compares ICD-10R diagnoses in those that met criteria for ASD (n=474) with those that did not (n=385). Rates of psychiatric diagnosis (>57%) were equivalent across both groups and exceeded general population rates for a number of conditions. The prevalence of anxiety disorders, particularly OCD, was significantly higher in adults with ASD than adults without ASD. Limitations of this observational clinic study which may impact on the generalisability of the findings include the lack of standardised structured psychiatric diagnostic assessments by assessors blind to ASD diagnosis and inter-rater reliability. The implications of this study highlight the need for careful consideration of mental health needs in all adults referred for ASD diagnosis.