Mast cells (MCs) always accompany connective tissue and are located in the proximity of lymphatic and blood vessels and nerve fibers. They are round or oval mononuclear cells with a diameter of 4-20 microm containing in their cytoplasm specific exocrine granules (storing neutral proteases) enclosed by a single membrane, whose presence is regarded as an index of the MC's static state. In view of their wide distribution in the organism, they play various roles in, for example, type I hypersensitivity reactions, chronic inflammatory processes, tissue reconstruction and wound healing, and pathological pulmonary fibrosis. They also play a role in angiogenesis, both in normal conditions during tissue regeneration and in pathological neoplastic states. The microcirculation provides building and nutritional substances to cancer cells and enables cancer spread via the blood. On the other hand, a tumor with good vascularization is more prone to penetration by cytostatics, which is why angiogenesis is a very important process in the course of neoplastic disease. Many authors indicate a close association between mast cells and angiogenesis. Some substances contained in the cytoplasm of these cells are potent stimulators of angiogenesis (tryptase, heparin), while others may inhibit it (protamine, platelet factor 4), and this conditions cancer growth and the development of the metastatic process. It is not known, however, what interactions occur between stimulants and inhibitors and what the proportional involvement of particular mediators in the formation of new vessels is.