A patient is described who presented with pancytopenia, splenomegaly and excessively elevated lactate dehydrogenase levels in concurrence with signs of extramedullary hematopoiesis. Although initially considered in the differential diagnostic spectrum, a highly aggressive lymphoma could not be identified before the patient died, 6 weeks after admission. Even an intensive diagnostic work-up including splenectomy and repeated bone marrow biopsies was inconclusive. Finally, the diagnosis of an intravascular large B-cell lymphoma, a highly aggressive clinical subtype of a diffuse large B-cell lymphoma, spreading within vascular structures of multiple organs was established by autopsy. Intravascular large B-cell lymphoma is often not diagnosed before death due to the exclusive intravascular growth pattern of the tumor cells and a fulminant clinical course. The heterogeneous clinical features of this lymphoma subtype are discussed.