PURPOSE: The purpose of this project was the development of a quality assurance (QA) system that would provide geographically accurate targeting for linac-based stereotactic radiosurgery (LBSR). METHODS AND MATERIALS: The key component of our QA system is a novel device (Alignment Tool) for expedient measurement of gantry and treatment table excursions (wobble) during rotation. The Alignment Tool replaces the familiar pencil-shaped pointers with a ball pointer that is used with the field light of the accelerator to indicate alignment of beam and target. Wobble is measured prior to each patient treatment and analyzed together with the BRW coordinates of the target by a spreadsheet. The corrections required to compensate for any imprecisions are identified, and a printout generated indicating the floor stand coordinates for each couch angle used to place the target at isocenter. RESULTS: The Alignment Tool has an inherent accuracy of measurement better than 0.1 mm. The overall targeting error of our QA method, found by evaluating 177 target simulator films of 55 foci in 40 randomly selected patients, was 0.47 +/- 0.23 mm. The Alignment Tool was also valuable during installation of the floor stand and a supplemental collimator for the accelerator. CONCLUSIONS: The QA procedure described allows accurate targeting in LBSR, even when couch rotation is imprecise. The Alignment Tool can facilitate the installation of any stereotactic irradiation system, and can be useful for annual QA checks as well as in the installation and commissioning of new accelerators.