OBJECTIVE An open, randomized study was performed to assess the effects of supportive pamidronate treatment on morbidity from bone metastases in breast cancer patients. METHODS Eighty-one pamidronate patients and 80 control patients were monitored for a median of 18 and 21 months, respectively, for events of skeletal morbidity and the radiologic course of metastatic bone disease. The oral pamidronate dose was 600 mg/d (high dose [HD]) during the earliest study years, then changed to 300 mg/d (low dose [LD]) because of gastrointestinal toxicity. Twenty-nine of 81 pamidronate (HD/LD) patients first received 600 mg/d and were then changed to 300 mg/d; 52 of 81 pamidronate LD patients received 300 mg/d throughout the study. Tumor treatment was unrestricted. RESULTS An overall intent-to-treat analysis was performed. In the pamidronate group, the occurrence of hypercalcemia, severe bone pain, and symptomatic impending fractures decreased by 65%, 30%, and 50%, respectively; event-rates of systemic treatment and radiotherapy decreased by 35% (P < or = .02). The event-free period (EFP), radiologic course of disease, and survival did not improve. Subgroup analyses suggested a dose-dependent treatment effect. Compared with their controls, in pamidronate HD/LD patients, events occurred 60% to 90% less frequently (P < or = .03) and the EFP was prolonged (P = .002). In pamidronate LD patients, event-rates decreased by 15% to 45% (P < or = .04). Gastrointestinal toxicity of pamidronate caused a 23% drop-out rate, but other cancer-associated factors seemed to contribute to this toxicity. CONCLUSIONS Pamidronate treatment of breast cancer patients efficaciously reduced skeletal morbidity. The effect appeared to be dose-dependent. Further research on dose and mode of treatment is mandatory.