The field service (FS) department constitutes, in many companies, an important organisation playing a vital role in their overall success. Field service managers require tools to analyse the impact of their decisions on the customer service level, which must be as high as possible (at least 95 %), and inventory cost, trying to reduce it to a minimum level (its ratio to the total FS revenue must be less than 10%). Such a tool is presented in this paper. We applied a closed queuing network model, the CAN_Q (computer Analysis of Networks of Queue) developed by Solberg. This model, although originally developed for modelling flexible manufacturing systems, has been applied to the FS departmentof a subsidiary of multinational computer company in Greece, and it has proved to be very efficient from the computational point of view and constitutes a powerfull tool for the FS managers, providing them with some useful performance measures. Its successful application opens new avenues of research, i.e., testing other queuing network models available from the literature, in combination with various inventory control models to help FS managers solve their critical problems.