On the basis of the histologic analysis of 45 solitary (juvenile) bone-cysts the author reveals new aspects of the pathogenesis. I. Data on sex-age-localisation are referred to. II. Due to the hemosiderosis regularly manifesting itself in the loose connective tissue layer lining the cyst and owing to the similarity to the liquified hematomas to be found elsewhere in the organism, as well as on the basis of the specific blood-supply conditions of the spongiosa of the increasing metaphysis the author presumes the intramedullary hemorrhage--possibly occurring also without fracture--to be the primary change. III. In the lining layer of the cyst, as well as in the adjacent bony tissue the author frequently observed capillary stase and to a considerable degree lymphedemas (resorption insufficiency). On the basis of these data the author presumes that the following mechanism might be hold responsible for the persisting of the cyst: The congestive (anoxic) wall of the capillary gets permeable for the proteins. Hence the fluid of high protein content, filling the cyst is constantly replaced. It is emphasized that solely the lymphatic vessels are apt to convey proteins. On the basis, however, of the histologic phenomena (lymphedema) observed, the pericystic lymphatic system is in the stage of relative insufficiency.