1. This is a preliminary report of an attempt to determine an objective reference point or "point of motion" during flexion and extension of the lumbar spine.

2. The method described uses superimposition of lateral radiographs taken in flexion and extension with the patient standing.

3. In seventy-eight radiographically normal subjects with no symptoms a "point of motion" was determined for each of the lowest three disc levels. At each level these points clustered within a specific zone approximately 2·5 centimetres square. Sixty-four per cent fell within a square centimetre.

4. In a comparative study of twenty-four patients with confirmed pathology, the "point of motion" fell outside the larger zone at the level of pathological change in 65 per cent of the disc levels.

5. The determination of the "point of motion" is a special technique for studying spinal motion. Its role as a diagnostic and prognostic aid in assessing patients with back pain is the subject of continuing study.