Inflammatory processes, tissue injury, and repair are normally locally balanced. However, continuing high levels of these phenomena can result in overt fibrosis. Increased numbers of mast cells (MC) are present in several fibrotic diseases occurring in the lung (such as idiopathic fibrosis, bleomycin or radiation induced fibrosis); in the skin (such as scleroderma, hypertrophic scars and keloids); and in the intestine (such as post-operative peritoneal adhesion and Crohn's disease). MC release a wide spectrum of biologically active compounds that, potentially, can be either fibrogenic or fibrolytic. In addition, in vitro studies in which MC or MC products were added to fibroblasts obtained from different organs, have demonstrated the profibrogenic properties of the MC. Moreover, some in vivo animal studies corroborate the in vitro findings on the importance of MC in fibrosis. This review provides strong evidence for the direct involvement of MC in fibrosis and tissue remodeling.