The surface of the large cystic spaces and dilated blood vessels in seven cases of aneurysmal bone cysts was studied. The endothelium of the blood vessels was surrounded by a layer of collagen types IV and V, and the endothelial cells contained factor VIII related antigen demonstrated by the peroxidase anti-peroxidase technique. Some blood vessels were dilated resembling small aneurysmal spaces. The visible surfaces of the aneurysmal spaces were devoid of collagen types IV and V, and of factor VIII related antigen. Ultrastructural analysis of paraffin embedded sections did not show the characteristic fine structural features of endothelium covering the aneurysmal spaces. It is concluded that the large spaces in aneurysmal bone cysts are devoid of basement membranes and endothelial cells. The absence of endothelium may explain the abundance of haemorrhages in these lesions. Immunocytochemical demonstration of endothelial antigen provides a valuable tool for the differential diagnosis between aneurysmal bone cysts and vascular tumours.