Psychiatric Clinical Knowledge Enhancement System (PSYCKES) is an innovative information system that presents patient medication history in tabular and graphical form. The system is designed to support therapeutic decision making. In this paper, we present a multifaceted cognitive evaluation of this system. The evaluation includes a cognitive walkthrough which is a task-analytic method for usability evaluation. We also conducted cognitive studies of two trainee and two attending psychiatrists using the system. One of the attending subjects is presented as a case study. An objective of this research is to characterize the way PSYCKES mediates reasoning. The study found that clinicians were able to use the system effectively to extract and coordinate information and draw appropriate inferences. The expert clinicians were better able to construct a coherent patient representation. The study also documented a few usability problems pertaining to the temporal integration of patient data. PSYCKES is a multifaceted tool that can significantly enhance therapeutic decision making.