This paper presents a novel method for assessing patients' perceptions of empowerment in the context of drug therapy, the Treatment-related Empowerment Scale (TES). The 10-item TES was specifically constructed to address components of communication, treatment choice, decision-making and satisfaction. Evaluation of the scale in a cross-sectional anonymous survey of 43 patients with advanced HIV infection revealed acceptable internal reliability (Cronbach's alpha=0.85) and evidence of both criterion and discriminant validity. Patients who perceived a high degree of treatment-related empowerment were less likely to view doctors as overly reliant on prescribing medicines and reported lower rates of intentional noncompliance. The TES has scope as a concise measure of patients' degree of control over the selection and use of drug therapy, and may be of particular value for current combination therapy regimens.