This cross-sectional study was designed to determine a cutoff score on the Apathy Evaluation Scale (AES) that predicts a clinician's designation of a subject with TBI as apathetic or not. Forty-five outpatients with TBI completed the AES-S, and 37 family members, friends, or significant others filled out the AES-I. Three clinicians prospectively gave their impressions of the presence or absence of apathy and retrospectively chose the degree of apathy on a 7-point subjective rating scale. The data was analysed by logistic regression and Receiver Operating Characteristic (ROC) curve. Sensitivity and specificity were calculated. No cutoff score on the AES-S or AES-I was found to have reasonable sensitivity and specificity with respect to the ability to predict the clinician's designation of a subject as apathetic. The AES requires further study if it is to be used to measure apathy following TBI.