Background: Ter beschikking stelling (TBS) clinics form the mainstay of forensic psychiatric services in the Netherlands. Their costs are rising, but little is known about how these costs are distributed. Aim: To determine the distribution of service costs for patients with personality disorders in TBS medium security units in the Netherlands. Method: Data on service use were extracted retrospectively from the case files of 55 people with personality disorder who are residents in six medium security units within two TBS centres during 2006 (De Rooyse Wissel and Pompestichting). Standard unit costs were obtained for each service, and multiplied by frequency of service use to obtain the total cost of service per patient. A modified version of the Secure Facilities Service Use Schedule was completed. Results: The average daily cost of a bed in a TBS hospital in 2006 was 388 Euros (402 (SD 37) Euros in De Rooyse Wissel; 375 (SD 48) Euros in the Pompestichting). Over half of this was spent on non-treatment fixed costs (overheads). There was considerable difference between patients and between unit variations in the other costs, but about one-third went on costs of staying in department (sociotherapists), and less than 10% each on specific therapeutic interventions or daily activities. About 3% of the budget overall was spent on other costs, but, as these included escorted leaves, at times these costs accounted for a much higher proportion of the per patient expenditure. Conclusion: Our results may provide a baseline measurement, with which future costs of TBS treatment can be compared as the services expand and develop. More attention to costs in this way may help to contain budget increases. New studies could examine more specific aspects of treatment or other specific patient groups.