Using a roentgen stereophotogrammetric method, the three-dimensional movements in the sacroiliac joints were quantified in 4 patients. To provoke motion of the sacrum, changes between body positions and a test with manual pressure were used. In tests with symmetric forces on the sacrum, it in most cases rotated mainly about a transverse axis and at most approximately 2 degrees. The axis of rotation passed through the iliac bones mainly in the lower part of the iliac tuberosity. The rotations between the iliac bones and the sacrum about any of the three main axes were determined with a precision in the mean of 0.2 degrees. The distance between the two superior posterior iliac spines varied at most 0.4 mm between seven different body positions.