Résumé : Patients with a spinal cord section loose a significant amount of bone. After paraplegia, bone loss occurs below the lesional level and is the more dramatic in iliac bones and in the metaphyseal area of long bones. A peak of urinary calcium and hydroxyprolinuria is observed approximately 6 weeks after their lesion. To further understand the mechanisms underlying the bone damage, we used long-term bone marrow cultures to compare osteoclast-like (OCL-like) cell formation above and below the lesional level. Seven paraplegic, one quadriparetic, one quadriplegic patients and five normal subjects were investigated. Six weeks after their spinal cord section, the number of OCL-like cells formed in iliac bone marrow cultures was significantly greater than those formed in sternal bone marrow cultures for all paraplegic patients tested. No significant differences were seen between iliac and sternal bone marrow cultures for the quadriparetic, the quadriplegic patient, or for the five normal subjects. Conditioned media (CM) from iliac marrow of paraplegic patients increased OCL-like cell formation in normal bone marrow cultures. IL-1, TNF-alpha, IL-6, and PGE2 were measured in the CM after 3 weeks of culture. IL-6 was found to be significantly higher in iliac CM compared with sternal CM in six out of seven paraplegic patients. In two patients, addition of an anti-IL-6 monoclonal antibody to the marrow cultures significantly decreased the number of OCL-like cells formed at 3 weeks. We conclude that paraplegia caused by a cord section locally induces an increase in the capacity of progenitors to form OCL-like cells in long-term bone marrow cultures. A locally increased IL-6 production in the marrow below the lesional level could be partly responsible for this observation.