Pelvic instability is a term widely used to describe a clinical entity characterized by pain localized to the pubic symphysis and/or sacroiliac joints. The pain often radiates down the leg. Hypermobility of the pelvic joints has been suggested as the cause of pain. Apart from several clinical stress tests, attempts at establishing objective evidence of such mobility have mainly consisted in radiologic measurement of motion of the pubic symphysis by a method which allowed only vertical movement to be recorded (5). In the present study symphyseal mobility was measured by an electromechanical method by which direct interosseous movement could be recorded in three directions and two different planes. Measurements were performed in a series of 15 patients with a tentative diagnosis of pelvic instability and the results compared to the results of radiologic measurement and to the normal values obtained in an earlier study of healthy volunteers (31). The results of this study would seem to refute the theory of symphyseal hypermobility being pathognomonic of so-called pelvic instability. It has moreover shown that radiologic measurement may be used for rough screening purposes, but is not sufficiently reliable for more exact measurements.