PURPOSE: To report the preliminary experimental results obtained with a robot-assisted transrectal ultrasound (TRUS)-guided prostate brachytherapy system METHODS AND MATERIALS: The system consists of a TRUS unit, a spatially coregistered needle insertion robot, and an FDA-approved treatment planning, image-registered implant system The robot receives each entry/target coordinate pair of the implant plan, inserts a preloaded needle, and then the seeds are deposited The needles/sources are tracked in TRUS, thus allowing the plan to be updated as the procedure progresses RESULTS: The first insertion attempt was recorded for each needle, without adjustment All clinically relevant locations were reached in a prostate phantom Nonparallel, parallel needle trajectories were demonstrated Based on TRUS, the average transverse placement error was 2 mm (worst case 2 5 mm, 80% less than 2 mm), and the average sagittal error was 2 5 mm (worst case 5 0 mm, 70% less than 2 5 mm) CONCLUSIONS: The concept, technical viability of robot-assisted brachytherapy were demonstrated in phantoms The kinematically decoupled robotic assistant device is inherently safe Overall performance was promising, but further optimization is necessary to prove the possibility of improved dosimetry