Systemic mastocytosis (SM) is a relatively rare heterogeneous group of disorders characterized by uncontrolled proliferation and accumulation of clonal mast cells in one or more organs. Indolent SM is the most common variety. Less common variety is aggressive systemic mastocytosis (aSM) type with or without associated hematological neopalsm (AHN). Fludeoxyglucose (FDG) positron emission tomography/computed tomography has a limited role in aSM without AHN as these exhibit low FDG avidity. We are presenting a biopsy-proven case of aSM without AHN showing abnormally high FDG uptake in lesions involving skin, nodes, marrow, and muscles.