On the basis of these evidences: a) the role played by heparin in the promotion of angiogenetic processes; b) the isolation of an angiogenetic factor in the synovial fluid from patients with rheumatoid arthritis; c) the significant increase of mast cells in the synovial membrane of the same patients, we suggest the possibility that the vasoproliferative processes in the course of rheumatoid arthritis may be mediated by heparin contained in the secretory granules of mast cells and released during the inflammatory process in response to numerous and heterogeneous stimuli.