Reliable measurement of student capability for a skill allows educators to verify student mastery. A major part of a physician's ability to gather information involves patient interviewing, and instruction of this skill is a substantial portion of a medical curriculum. Since 1974, the University of Arizona College of Medicine has employed patient-instructors (PIs), lay persons who function in the roles of patient and teacher for training of interview skills in the Preparation for Clinical Medicine (PCM) program. PIs provide "real" patient-interview experiences and immediate feedback to the students. The PCM program currently has four topic areas: Adult, Pediatric, Geriatric, and Psychiatric. The Arizona Clinical Interview Rating (ACIR) Scale was developed in 1976 to measure the technical performance aspects of interviewing. This study was undertaken to determine reliability of the ACIR. Implication of the results are discussed and suggestions made for the continued application of the ACIR Scale. (Abstract shortened with permission of author.)