Studies of a 63-year-old woman with osteopenia due to systemic mastocytosis are reported. Delay in making the diagnosis and the occurrence of coincident medical problems meant that radiographs during a 12-year period were available for study. Skeletal symptoms began 20 years after the skin lesions of mastocytosis first appeared, but once established bone loss was rapid, particularly from cortical bone (estimated at 3.75% pa). Biochemical and histological measurements indicated a "high turnover" bone disease and treatment with inhibitors of bone resorption was assessed. Oral, but not intravenous, clodronate was effective in reducing bone turnover as judged by falls in hydroxyproline excretion (p less than 0.01) and serum alkaline phosphatase (p less than 0.01). Mithramycin and chlorambucil were ineffective. Clodronate may be beneficial in arresting bone loss in this disorder.