Background. - Borderline personality disorder (BPD) is a highly prevalent, chronic condition. Because of its very problematic nature BPD is expected to be associated with substantial societal costs, although this has never been comprehensively assessed. Objective. - Estimate the societal cost of BPD in the Netherlands. Study Design. - We used a prevalence-based bottom-up approach with a sample of 88 BPD patients who enrolled in a multicenter clinical trial comparing two kinds of outpatient psychotherapy. Costs were assessed by means of a structured interview, covering all healthcare costs, medication, informal care, productivity losses, and out-of-pocket expenses. Only BPD-related costs were included. All costs were expressed in Euros for the year 2000. A bootstrap procedure was performed to determine statistical uncertainty. Patients. - All patients had been diagnosed with BPD using DSM-IV criteria. Mean age was 30.5 years and 92% was female. Results. - Based on a prevalence of 1.1% and an adult population of 11,990,942, we derived that there were 131,900 BPD patients in the Netherlands. Total bootstrapped yearly cost of illness was is an element of 2,222,763,789 (is an element of 1,372,412,403-is an element of 3,260,248,300), only 22% was healthcare-related. Costs per patient were is an element of 16,852. Conclusions. - Although healthcare costs of non-institutionalized Borderline patients might not be disproportionate, total societal costs are substantial. 2007 Elsevier Masson SAS. All rights reserved.