Our purpose is (1) to present 5 cases of hemangioma of the knee joint in which the condition has been diagnosed and treated by us, (2) to set forth the criteria for an accurate diagnosis and the most favorable methods of treatment and (3) to analyze the cases reported in the literature. The peculiar nature of this tumor, the frequency of admitted error in diagnosis and the relative lack of success in its treatment make the 5 cases intensely interesting. The following series of 5 cases of hemangioma of the knee is the largest reported from any one clinic. To our knowledge radium and roentgen therapy have not been used before in the treatment of this condition. In 2 cases the patients were treated by the accepted methods prior to the use of roentgen therapy. In 3 cases treatment with roentgen rays and radium as suggested by one of us