Numerous research and development projects have been aimed at implementing computerized patient record (CPR) systems. Yet little emphasis has been placed on physicians' ability to learn and use these systems or on their effects on physicians' reasoning. This article describes an innovative approach to assessing these aspects of a CPR system. The method involves observing physicians' use of a CPR under various clinical conditions and analyzing the CPR system with a technique called the cognitive walkthrough. We will show how learning to use a CPR system can change a physician's performance, with accompanying effects on information gathering and reasoning.