For certain external beam radiotherapy procedures, precise alignment of patients with the treatment beam is essential for good treatment outcome. A method has been developed for quickly achieving precise patient alignment with the aid of stereoscopically located fiducial markers. The alignment algorithm is developed from standard rigid body mechanics using closed form solutions, obviating the need for iterative fitting methods. The technique is implemented with a digitizing tablet and plane film radiographs. The accuracy of alignment with this method in phantom studies is better than 1 mm and 1 deg relative to CT data. The repeatability of positioning is 0.5 mm (standard deviation) and 0.36 deg (standard deviation).