The presence of an hemarthrosis generally is a sign of a particularly severe joint lesion. Arthrous alterations appearing post-traumatically can therefore be due to the extent of the joint lesion as well as to the accompanying hemarthrosis. A group of 24 young adult rabbits was submitted to morphologic examinations in order to find out in which way an isolated hemarthrosis can be responsible of degenerative cartilage alterations. At intervals of eight days each, 3 cm3 of their own blood were injected into both knee joints of the rabbits. One knee joint was immobilized, the other was mobile. The results prove that the hemarthrosis represents a considerable risk with regard to the trophism of the concerned joint. A direct enzymatic damage to the joint cartilage alone or in combination with trophical alterations appearing later on can lead to a formation and aggravation of arthrosis. The degenerative alterations are increased in an immobilized joint. Bearing in mind these alterations, the conclusion is drawn for clinical practice that the hemarthrosis has to be considered when establishing both diagnosis and therapy plan (punction, joint lavage, drainage, immobilization as early as possible).