The need for orthodontic treatment has an objective component based on occlusal traits and a subjective component based on the esthetic impact of the occlusion. An occlusal index that measures the objective deviation from normal or ideal occlusion might be sufficient to mirror the subjective opinion of orthodontists about treatment need. The objective of this study was to determine whether the American (US) and United Kingdom (UK) weightings of the peer assessment rating (PAR) index are valid instruments with which to determine treatment need. Fifteen orthodontists rated the need for orthodontic treatment of 170 casts. Their collective decision was compared with the PAR value for the cast determined by a calibrated examiner. A range of suggested treatment cutoff points from the literature was used to generate receiver operating characteristic (ROC) curves and optimized cutoff points. The cutoff points were 17 for both the US PAR and the UK PAR, and sensitivity, specificity, and kappa were 92%, 86%, and 0.77 for the US PAR and 92%, 89%, and 0.80 for the UK PAR. The area under the ROC curve was 97% for the US PAR and the UK PAR. Both the US PAR and the UK PAR scores were excellent predictors of orthodontic treatment need as determined by a panel of orthodontists. An occlusal index used to measure deviation from normal or ideal occlusion might perform as well as indexes of treatment need in predicting orthodontists' evaluations of treatment need.