Pelvic instability is a controversial concept. The symptoms and signs are believed to be due to pelvic joint hypermobility. Differentiation from low back pain can often be difficult. As an aid in diagnosis, temporary external fixation of the pelvis with a trapezoid compression frame was used in 12 patients. The effect on clinical signs and limping, tested on an electronic walkway, and symphyseal mobility has been analysed: in 11 of the 12 patients the frame had a positive clinical effect. In five of six patients limping diminished significantly. Stabilizing of symphyseal mobility could not be demonstrated.