Ultrasound-guided transperineal prostate implantation is a new technique for performing permanent isotope implants of the prostate. The details of the technique are presented to demonstrate its ability to place radioactive seeds three-dimensionally within the prostate gland to achieve uniform dose distribution without the need for complicated preplanning. An accurate measurement of the prostate volume is made using biplanar transrectal ultrasound. The total activity to be implanted is derived from a look-up table based on prostate volume. The basic plan is to implant 60-70% of the total activity in the periphery of the gland and the remaining activity in the interior of the gland. The ultrasound transducer provides visualization of the prostate through transverse, longitudinal and oblique cuts and allows for accurate placement of implant needles, approximately 1 cm apart. In addition, these needles can be moved through the prostate under constant visualization, thus allowing for precise seed placement. The setup of the transrectal ultrasound device as well as prostate volume measurements are performed in 10 to 15 min. The actual placement of the needles and seed implantation takes 1 to 1.5 h to perform. Postimplantation dosimetric evaluation is performed using orthogonal x-ray films and 3 mm thick CT slices taken at 3 mm intervals. This evaluation has confirmed accurate seed placement within the prostate gland. Interactive ultrasound guided transperineal prostate implantation is a fast and accurate method of performing permanent radioactive isotope prostate implants.