Theoretical and experimental methods were used to evaluate a design of ultrasound applicators for interstitial hyperthermia. The basic schema consists of a multielement array of tubular piezoceramic radiators (1.5-1.6 mm diameter), each 5-10 mm long with separate power control, designed to be inserted within a 13-14 gauge closed-end brachytherapy implant catheter. Channels for circulating temperature-regulated water are integrated within the applicator to provide control of the catheter/tissue interface temperature. A quantitative theoretical analysis was undertaken to determine heating performance as a function of applicator spacing, blood perfusion, catheter material, frequency and required acoustic power output. Prototype multielement applicators were constructed and characterized in terms of acoustic pressure-squared distributions and acoustic power output capabilities. This study demonstrated distinct advantages of these catheter-cooled multielement ultrasound applicators within an implant, including higher T90's achievable within highly perfused tissues, dynamic control of the longitudinal power deposition, and no interaction between adjacent applicators or elements or sensitivity to applicator alignment. Preliminary measurements of prototype devices have indicated that implementation of these interstitial ultrasound applicators with integrated catheter-cooling is practicable, yet further development and in vivo verification of performance is warranted.