Thirty-eight normocalcemic patients with bone metastases from breast carcinoma were randomized to receive dichloromethylene diphosphonate (CL2MDP) in addition to their specific antitumor treatment (chemotherapy and/or hormone therapy), at a dose of 300 mg/day/i.v. or placebo for the first 7 dys. TheCL2MDP treatment then continued at a dose of 100 mg/day/i.m. for 3 weeks and finally at 100 mg/i.m. on alternate days for at least another 2 months. In both groups of patients there was a reduction in the intensity of pain (Scott-Huskisson analog), but there was a more frequent reduction in the daily consumption of analgesics in patients treated with CL2MDP (p = 0.02). Unlike the controls, the patients who received CL2MDPpresented a significant reduction in urinary calcium (p = 0.003) and in hydroxyproline (p = 0.05) on the 7th day. As regards the clinical evolution, negative events such as the appearance of hypercalcemia, pathological fractures, new bone lesions or a substantial increase in the preexisting ones, were observed in 9 of the 12 evaluable patients treated with placebo and in 3 out of 9 treated with CL2MDP. Thickening of the preexisting osteolytic lesions was reported in 2 patients treated with CL2MDP. Tolerance was excellent: only a few patients complained of pain at the intramuscular drug injection site.