Background: To date, numerous cost-of-illness studies have been using methodologies that don't provide trustworthy results for decision making in mental health care. Objectives: The aims of this paper are design and implement a cost methodology by process of patient's care to calculate unit costs in mental health in Spain in 2005 and compare the results with the reached ones by traditional methods. Methods: We adapted Activity-Based-Costing to this field analyzing the organizational and management structure of Mental Health's public services in a region of Spain, Navarre, describing the processes of care to patient in each resource and calculating their cost. Results: We implemented this methodology in all resources and obtained unit cost per service. There are great differences between our results and the ones calculated by traditional systems. We display one example of these disparities contrasting our cost with the reached one by the methodology of Diagnostic Related Group (DRG). Conclusions: This cost methodology offers more advantages for management than traditional methods provide.