We have compared in an open trial the clinical and biochemical effects of a new aminodiphosphonate, aminohydroxybutylidene diphosphonate, with those of dichloromethylene diphosphonate, which has been proved effective. The patients presented extensive and symptomatic bone involvement from multiple myeloma, breast cancer, and other metastatic tumors. The treatment consisted of aminohydroxybutylidene diphosphonate, 2.5 mg/d intravenously for five days, or dichloromethylene diphosphonate, 300 mg/d intravenously for seven days, followed by 100 mg/d intramuscularly for ten days. Twelve patients treated with aminohydroxybutylidene diphosphonate and 16 patients treated with dichloromethylene diphosphonate were assessable and were followed up for one to six months. Therapy with aminohydroxybutylidene diphosphonate showed a quicker action in reducing bone pain and reduced significantly more the serum calcium level than did therapy with dichloromethylene diphosphonate. Aminohydroxybutylidene diphosphonate therapy also affected urinary calcium levels and hydroxyproline excretion more markedly than did dichloromethylene diphosphonate, although the differences are not statistically significant. However, the biochemical indexes rebounded more quickly in patients treated with aminohydroxybutylidene diphosphonate, indicating that the loading amount (only 12.5 mg) used in this preliminary study is insufficient to sustain a prolonged effect. The effectiveness and lack of side effects render aminohydroxybutylidene diphosphonate an attractive treatment for malignant bone resorption.