A rare form of mast cell leukemia characterized by the presence of at least 20% mast cells in bone marrow aspirate smears but often mature mast cell morphology, low proliferation rate, and absence of organ damage and C findings (cytopenias, hepatomegaly, ascites, portal hypertension, splenomegaly, skeletal lesions, malabsorption). The disease course is less aggressive than in the acute form, although patients may later progress.