Although mast cell proliferation in the bone marrow frequently occurs in systemic mastocytosis and is often associated with radiographically detectable bone lesions, the pathologic correlates of the skeletal abnormalities are poorly characterized. We therefore examined three nondecalcified transiliac crest biopsy specimens from two patients with systemic mastocytosis and diffuse osteopenia with vertebral crush fractures. Marrow involvement included unusual mast cell aggregates, as revealed by metachromatic staining, that mimicked granulomas. Histomorphometric analysis of trabecular bone revealed accelerated bone remodeling or "turnover" characterized by osteoidosis, peritrabecular fibrosis, increased numbers of osteoblasts and osteoclasts, and an increase in osteoclastic resorbing surfaces. Our observations and review of the literature suggest that with the recent development of techniques for assessing undecalcified bone biopsy specimens, mastocytosis will probably be shown to be a more common etiology in "osteoporosis" than previously recognized. Metachromatic staining of the biopsy specimen should be a routine procedure in the investigation of any patient who undergoes diagnostic bone biopsy.