The clinical and research experience of para-osteo-arthropathies (POA) observed in paraplegia is presented. From clinical findings and our own research studies, a new theory regarding the etiopathogenesis of the ectopic ossifications has been elaborated. POA is a biological process in which new bone is formed in tissues which in the normal condition do not ossify. A 45Ca kinetic study showed that POA are young bones with an active 'turnover'. The urinary hydroxyproline level, already increased from the onset of the paraplegia, rises at the moment of POA formation. On the other hand HLA B 18 and HLA RW 7 antigens are significantly increased in patients with POA. Several factors such as vascular disorders (venous stasis, modification of gaseous exchange) repeated microtraumatism at the site of the joints, injured perimuscular connective tissue, as well as increased incidence of some antigens, can play a role in the formation of POA. Metabolic changes related to vascular disorders probably influence cell differentiation, mostly the differentiation of fibroblasts in the perimuscular connective scar tissue following repeated microtraumatism. On the basis of observations the authors have tried to devise a treatment. Prevention remains the best way to avoid ectopic ossification but, once present, medical and surgical treatment must be undertaken.