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BACKGROUND This study evaluated the dose–response relation for zoledronic acid, a new generation high potency bisphosphonate, given as a 5-minute infusion in patients with malignant osteolytic disease. METHODS Two-hundred eighty patients with osteolytic lesions due to metastatic breast carcinoma or multiple myeloma were randomized to double-blind treatment with either 0.4, 2.0, or 4.0 mg of zoledronic acid or 90 mg pamidronate. The primary efficacy endpoint was the proportion of patients receiving radiation to bone. Other skeletal-related events, bone mineral density (BMD), bone markers, Eastern Cooperative Oncology Group performance status, pain and analgesic scores, and safety also were evaluated. RESULTS Zoledronic acid at doses of 2.0 and 4.0 mg and pamidronate at a dose of 90 mg each significantly reduced the need for radiation therapy to bone (P < 0.05) in contrast with 0.4 mg zoledronic acid, which did not. Skeletal-related events of any kind, pathologic fractures, and hypercalcemia also occurred less frequently in patients treated with 2.0 or 4.0 mg zoledronic acid or pamidronate than with 0.4 mg zoledronic acid. Increases in lumbar spine BMD (6.2–9.6%) and decreases in the bone resorption marker N-telopeptide (range, −37.1 to −60.8%) were observed for all treatment groups. Skeletal pain, fatigue, nausea, vomiting, and headache were the most commonly reported adverse events. Adverse events were similar in nature and frequency with zoledronic acid and pamidronate. CONCLUSIONS A 5-minute infusion of 2.0–4.0 mg zoledronic acid was at least as effective as a 2-hour 90-mg pamidronate infusion in treatment of osteolytic metastases. A 0.4-mg dose of zoledronic acid was significantly less effective. Both zoledronic acid and pamidronate were well tolerated. Cancer 2001;91:1191–200. © 2001 American Cancer Society.