In the vast majority of patients with systemic mastocytosis (SM), the bone marrow is the primary extracutaneous site of disease. In addition to bone marrow involvement, other visceral organs such as the spleen, liver or the gastrointestinal tract, may also be affected. However, isolated involvement of a single extramedullary organ is rarely seen in SM. We report on two patients with SM with splenic involvement, lack of 'diagnostic' mast cell (MC) infiltrates in the bone marrow, and absence of skin lesions. In one patient, a myelodysplastic syndrome was diagnosed prior to the detection of SM. Both patients presented with massive splenomegaly and multifocal MC infiltrates in splenic tissues. These MCs also expressed CD25 as well as the C-KIT mutation D816V. In consecutive examinations, the mutation was also detected in the bone marrow in both patients suggesting diffuse infiltration with neoplastic cells. In summary, our data show that the spleen can be a primary site of disease in rare cases of SM. Mastocytosis should therefore be considered as a (rare) differential diagnosis in patients with splenomegaly of unknown etiology.