Abstract: Volume rendering applications are used in medical imaging in order to facilitate the analysis of three-dimensional image data. This study focuses on how to improve the usability of graphical user interfaces of these systems, by gathering user requirements. This is achieved by evaluations of existing systems, together with interviews and observations at clinics in Sweden that use volume rendering to some extent. The usability of the applications of today is not sufficient, according to the users participating in this study. This is due to a wide range of reasons. One reason is that the graphical user interface is not intuitive. Another reason is that the users do not rely on the technique to produce sufficient results that can be used in the diagnostic process. The issue of user confidence is mainly due to the problem of the generation and user control of the transfer functions used in volume rendering. Based on the results of the evaluation a graphical user interface, including the most important and frequently used functions, is designed. A suggestion for how the transfer function can be generated is presented.