The aim of this study was to assess the variation in film and patient positioning in horizontally corrected sagittal tomography of the temporomandibular joint (TMJ). The influence of this variation on linear and angular measurements of some anatomic structures of the TMJ was also studied. There was no significant variation in film positioning in the tomograph, using a multi-film cassette. The variation in positioning the patient in the tomograph was significant when four dental assistants were asked to place the Frankfort plane parallel to the horizontal plane. The measurements with the film in a straight position were compared with the measurements with the film angulated, to simulate the mean variation in film and patient positioning. Linear and angular measurements of anatomic structures were performed in arthrotomograms of 58 joints, representing joints with superior disk position and joints with anterior disk position with and without reduction. There was a difference between the two measurements for four different distances and one angle independent of diagnosis. No differences could be found between patients belonging to the different diagnostic groups. The results indicate that variation in patient positioning influences linear and angular measurements of anatomic structures in TMJ arthrotomograms.