A 63-year-old woman presented with a short history of progressive lumbosacral pain. Conventional radiography of the spine and pelvis (top left) showed multiple osteolytic lesions. T1-weighted magnetic resonance imaging confirmed disseminated osteolytic lesions, showing an inhomogeneous structure of the spinal column (top right) and the pelvic bone. In the light of these results, metastatic disease was suspected and the patient was admitted to our hospital for diagnosis and further management. The cause of the osteolytic lesions was elucidated by bone marrow biopsy. The May-Grünwald-Giemsa stained cytological smears showed a normocellular marrow with a diffuse infiltration by spindle shaped mast cells. There were focal clusters of mast cells detectable (bottom left), which expressed CD2 and CD25 by immunostaining. Consistent with the mast cell phenotype, cells were strongly positive for chloracetate esterase and toluidine-blue (bottom right). A diagnosis of systemic mastocytosis was confirmed by an elevated serum tryptase level (134 lg/l, normal: <11 lg/l) and the detection of the D816V KIT mutation.