A CT-guided technique for prostate brachytherapy has been developed which can be performed on patients with large prostates and which allows real time evaluation and modification of implant geometry. The patient is positioned prone and radioactive Pd 103 or I 125 seeds are implanted through the transischiorectal space. Needles are inserted through a template whose plane is oriented at a nominal angle of 26 degrees away from the horizontal to facilitate needle penetration through the ischiorectal space. The CT gantry is tilted 26 degrees, so that its plane is orthogonal to that of the template. The oblique geometry between the CT gantry and direction of couch translation must be considered during source position planning, implantation, and post-treatment evaluation. This consideration is presented along with discussion of relevant quality assurance procedures and recommended tolerances. The mechanical and radiological tolerances of the CT scanner must be consistent with the high level of precision required in radiation therapy. Special emphasis was placed on gantry laser and image plane alignment, sensitivity and radiation profiles, and spatial accuracy of image reconstruction, table translation, and gantry tilt.