The syndrome of mastocytosis extends from cutaneous urticaria pigmentosa through systemic mastocytosis to the rarely occurring mast cell leukaemia. Our investigations with a large patient collective have shown that systemic forms of the disease occur more often than is generally supposed. In particular the incidence of bone marrow manifestations deserve more attention. The inflammatory-granulomatous findings in the bone marrow suggest an immunoactive component in the pathogenesis of this disease. The bone lesions that occur in about 50% of patients are probably the result of the common endosteal site of the mastocytosis granuloma. These lesions can be generalized (osteoporosis-osteosclerosis) or localized (osteolytic-osteosclerotic foci). In clinical practice bone biopsy and skeletal radiology complement one another; in addition to skin biopsy bone biopsy supplies the initial diagnosis of mastocytosis and documents systemic manifestation; the X-ray picture informs the clinician about the type and extent of the bone pathology.