To date, the diagnosis of mast cell disease (MCD) relied on routine plus histochemical stains. Its differential diagnosis, however, includes a variety of other hematopoietic and particularly B-cell lymphoid neoplasms that are best identified in paraffin sections using immunostains. To determine the paraffin-section immunoreactivity of MCD, 20 specimens from 14 patients with MCD and 1 bone marrow sample (from a patient with probable MCD) that showed equivocal metachromasia, were stained with antitryptase, CD68 (KP-1), CD20 (L26), antilysozyme, and antimyeloperoxidase antibodies. Ten hairy cell leukemias (HCLs), six lymphomas of parafollicular and/or monocytoid B-cell (MBCLs) and low-grade mucosa-associated lymphoid tissue (MALT) types, six granulocytic sarcomas, and five acute myeloid leukemias with monocytic differentiation (M4 and M5 types) were also stained. Tryptase positivity was identified in all of the MCD cases. The staining was moderate to strong in 20 of the 21 specimens, including the probable MCD case. No other neoplasms tested were tryptase positive. CD68 showed similar to even stronger staining in all of the specimens of MCD, HCL, granulocytic sarcoma, and acute myeloid leukemia (M4 and M5 types) tested and in five of the six MBCL and/or MALT-type lymphomas. Weak-to-moderate lysozyme staining seemed to be present in at least 7 of the MCD specimens, whereas there was a lack of staining for myeloperoxidase in 12 specimens, and 7 specimens were nonevaluable (1 case was not tested). Myeloperoxidase was identified in all of the granulocytic sarcomas and acute myeloid leukemias (M4 and M5 types) but not in any HCLs, MBCLs, or low-grade lymphomas of MALT type. CD20 was negative in all of the MCD and myelomonocytic neoplasms but positive in all of the HCLs, MBCLs, and low-grade B-cell lymphomas of MALT type. MCD, therefore, has a characteristic tryptase-positive, CD68-positive, and CD20-negative phenotype in paraffin sections. This distinguishes MCD from the hematopoietic and/or lymphoid disorders that it most closely resembles.