Malignant Systemic Mastocytosis is a very rare condition. Only about less than 40 well documented cases have been reported as per the available literature. The paper presents the case report of a 54 year old male patient who presented with huge hepatosplenomegaly and abdominal lymphadenopathy. Splenectomy specimen was 17 x 16 x 10 cm size with cut surface studded with numerous tiny 1-2 mm nodules. Histologic sections of spleen showed extensive mast cell (typical and atypical) infiltrates. Liver biopsy and abdominal lymphnode biopsy specimens and bone marrow smears also showed similar infiltration by mast cells. Special stains done for non-specific esterase and chloracetate esterase showed strong positivity for mast cells. The results of immunohistochemical and electron microscopic studies are also presented.