The authors undertook a morphometric study of the intervertebral foramina in anatomic specimens of spines unaffected by degenerative lesions. They studied the variations in shape and size of 38 intervertebral foramina during flexion-extension movements and following an isolated disc collapse. The cast technique takes into account the bony prominences, the bulge of the disc anteriorly and of the capsulo-ligamentous structures posteriorly. In flexion, all the diameters of the foramina are maximal. In full extension all the diameters decrease significantly: the pedicles come closer together, the disc bulges posteriorly and the ligamentum flavum is pushed forward by the superior articular process of the underlying vertebra. A disc collapse of 4 mm decreases all the diameters, and in this case lumbar extension results in a sufficient decrease of foraminal diameter to threaten the nerve root.