Evaluation of the results of orthodontic treatment is crucial for the improvement of the practitioner's therapeutic skills. Conventional methods of appraisal are based on illdefined subjective criteria, however, or fail to make their criteria explicit. In order to provide a more objective appraisal, we surveyed 10 experienced practitioners regarding their criteria for evaluating 12 items such as the overjet, or the amount of horizontal projection of the upper front teeth beyond the lower ones. For each item, their opinions were averaged as the definition of membership in the category of "completely favorable." Algorithms were devised for comparing the posttreatment results with the pretreatment condition of need, in order to evaluate the degree of therapeutic success. The average of all 12 evaluations is taken as the appraisal of treatment. A hardware system incorporating these algorithms has been developed. This system, representing the first successful application of fuzzy inference to the evaluation of orthodontic treatment, is expected to provide orthodontists with an accurate and objective means of making such an appraisal and thereby help improve their therapeutic skills.@ 1994 John Wiley & Sons, Inc.