Introduction: The available information on the cost of illness of Borderline Personality Disorder (BPD) is overtly insufficient for policy planning. Our aim was to estimate the costs of illness for BPD in Catalonia (Spain) for 2006. Methods: This is a multilevel cross-design synthesis study combining a qualitative nominal approach, quantitative 'top-down' analysis of multiple health databases, and 'bottom-up' data of local surveys. Both direct and indirect costs have been estimated from a governmental and societal perspective. Results: Estimated year-prevalence of BPD was 0.7% (41,921cases), but only 9.6% of these cases were treated in the mental health system (4033cases). The baseline of the total cost of BPD in Catalonia was 45.6million. €, of which 15.8million. € (34.7%) were direct costs related to mental health care. The cost distribution was 0.4% in primary care; 4% in outpatient mental health care; 4.7% in hospitalisation; 0.7% in emergency care; and 24.9% in pharmacotherapy. Additionally, the cost of drug addiction treatment for persons with BPD was 11.2%; costs associated with sheltered employment were 23.9% and those of crime and justice were 9.7%. Indirect costs -including temporary sick leave and premature death (suicide)- represented 20.5% of total costs. The average annual cost per patient was 11,308. €. Conclusions: An under-reporting of BPD was identified by the experts in all health databases and official registries. Most of the BPD costs were not related to mental health care. Amongst the direct cost categories, pharmacotherapy had the largest proportion despite the lack of specificity for BPD. This distribution of costs reinforces the idea of BPD complexity related to an inadequate and inefficient use of health resources.