Bone radiological signs in six cases of systemic mastocytosis are described: lesions may be diffuse or circumscribed and of the condensation and/or rarefaction type. In most cases, however, the lesions are of the mixed type. Bone mastocytosis may occur without any cutaneous lesions and this emphasizes the value, from the diagnostic point of view, of radiological examinations of the skeleton. Diagnosis is defined and confirmed by three features: the association of condensing lesions and bone rarefactions in different regions of the skeleton; the presence of mastocytic proliferation in stained biopsy specimens from the posterior iliac bone which is not decalcified; fixation of 99m tc pyrophosphate in the axial skeleton on scintigraphy.