Mastocytosis is a disease of disordered mast cell proliferation. It affects all ages, both sexes, and all ethnic groups. In some cases, mastocytosis has an aggressive and ultimately fatal course. Thus, our efforts are directed to improving diagnosis and treatment; and to identifying the etiology of this disease. Most recently we have described pulmonary, abdominal, and ovarian manifestations of aggressive mastocytosis. A successful approach to treatment for aggressive disease remains elusive. Three patients treated for one year or more with interferon alpha-2b (IFNa-2b) continued to show a progressive course. However, in vitro studies demonstrate that IFNg-1b does suppress human mast cell growth, where IFNa-2b does not. Mast cell releasibility is not increased by IFNg-1b. Studies on c-kit and the relevance of activating mutations which we first identified in mastocytosis patients are continuing. Data now indicates all patients with mastocytosis and an associated hematologic disorder have the point mutation Asp816 Val in peripheral blood mononuclear cells. In contrast, adult patients with indolent mastocytosis may demonstrate this mutation only in skin lesions. Molecular studies are consistent with the conclusion that the Asp816 Val mutation is a somatic mutation and is not in germ line tissues. The search for other activating mutation continues.