Very few medical knowledge based systems have undergone evaluation in the operational environment. An important reason for the limited number of reported field evaluations is the fact that extensive laboratory evaluation will have to precede the evaluation of the systems in field. The aspects of field evaluation which will be considered in this paper are the man-machine system and the interaction between the user and the system. This will include a discussion of the field evaluation of the knowledge based system PLEXUS. PLEXUS is a medical knowledge system, designed to assist physicians with the diagnosis and treatment planning of nerve injuries in the neck.