The diagnosis of personality disorder (PD) in individuals with an intellectual disability (ID) is clinically relevant, but there is little research on the subject. Because the impact of PD on both patients and carers is substantial, it is important to increase the understanding of how ID and PD co-occur.

The aim of the present study was to examine the prevalence of all axis II DSM-IV-TR PD diagnoses in a well- defined sample of ID out-patients in two specialized secondary care ou-patient treatment centers, in the Netherlands. Special attention was paid to borderline ID (70≤Total IQ < 85), prevalence of co-morbid axis I disorders and associations of PD with several demographic variables.

The present study was a cross-sectional medical chart review. On January first, 2011, 599 patients were registered at the two centers. Diagnostic information was available for 576 patients. Diagnoses were based on Došen's integrative approach and formulated according to the DM-ID.

194 patients (33.7%) were diagnosed with a PD. PD NOS (19.1%) was most common, followed by borderline PD (8.7%). Patients with borderline ID were most likely to be diagnosed with a PD (43.7%). 85.1% of patients diagnosed with a PD had at least 1 co-morbid axis I diagnosis. PTSD was the most common co-morbid disorder (23.7%).

Diagnosed according to strict criteria, PDs often occur in ID out-patients, mostly together with axis I disorders. Importantly, PDs and PTSD often co-occur in this group, indicating special care needs and signifying an interesting direction for further research.