A case of malignant mastocytosis with peripheral blood involvement is presented. The course of the patient's illness was complicated by recurrent hypotensive episodes, presumed to have been caused by mast cell degranulation. Treatment with hydroxyurea was associated with persistent hypotension which resulted in death. It has been proposed that the diagnosis of mast cell leukemia be given to patients presenting with greater than 10% atypical mast cells in the blood. However, review of 16 published cases of malignant mastocytosis with circulating mast cells reveals that the clinical manifestations, complications, and survival do not vary significantly with the percentage of peripheral blood mast cells. Patients with malignant mastocytosis with significant involvement by atypical mast cells in the bone marrow and peripheral blood should be considered as having an aggressive disease, regardless of the percentage of circulating mast cells.