Thirty two cases of the association of mastocytosis and bone lesions were collected in a multicentre study. Five cases involved osteocondensation forms. However, most often (27 cases), there was osteoporosis (OP). The diagnosis was made in the absence of obvious risk factors, and thus often in men (2/3 of patients), when there was the association of pigmented urticaria and an excess of mast cells in bone biopsies. Laboratory, radiological and isotope scan findings are often non-specific, being identical to those encountered in common OP. The histomorphometric profile involves an association of decreased cancellous bone volume, increased area of resorption and decreased bone formation parameters. Progression to malignant mastocytosis occurs essentially in diffuse osteocondensation forms and is rare in OP types. Emphasis must be placed on the importance of qualitative study of bone marrow, using specific stains, since the diagnosis may be missed in the absence of typical skin lesions. Conversely, since a simple increase in mast cell count is possible during common OP, a search for mast cell nodules is important in order to establish the diagnosis with certitude.