Various radiological parameters were measured on a lateral film taken in erect position of the lumbar spine in 55 patient with low back pain. These various parameters had inter-, intra-observer and inter-study variation coefficients of less than 3%. In patients with a positive Fergusson index ("low" or inferior lordosis), mean values of the angle of lordosis and of obliqueness of the lower vertebral plateau of L5 in relation to the horizontal were higher than in patients with a negative Fergusson index ("high" or superior lordosis), this being statistically significant. In view of the correlations found between the various parameters measured and this classification, the biomechanical consequences, which have their role to play in the management of patients with low back pain, are discussed.