Bisphophonates are the treatment of choice to prevent skeletal events in patients with multiple myeloma. Some preclinical studies suggested that bisphophonates can be useful as antitumor drugs in some malignancies. We conducted a controlled clinical trial to assess if zoledronic acid can have this clinical activity. Ninety four patients with previously untreated multiple myeloma were treated with a conventional chemotherapy program: cyclophosphamide, vincristine, melphalan, and prednisone (CVMP) and were randomized to received either zoledronic acid (4 mg, iv, every 28 d) or not (control group). The end-point of the present study was to assess improvement in outcome, measured by event-free survival (EFS) and overall survival (OS), and the second-end point was to confirm the efficacy in preventing skeletal events. In an intent-to-treat analysis, all patients were available for efficacy and toxicity. Median follow up was 49.6 mo (range: 34-72 mo). Five year actuarial curves showed that EFS was 80% in the zoledronic acid group, which was statistically different from 52% in the control group (p < 0.01). Actuarial 5 yr OS was 80% in the zoledronic acid arm, and 46% in the control group (p < 0.01). Sketeletal events were more frequent in the control group when compared to zoledronic acid. Toxicity was mild. We confirm the efficacy of zoledronic acid to prevent skeletal events, but we felt that we can demonstrate that zoledronic acid has a clinical antitumor effect measured from a increase in complete response rate and EFS and OS that were better when compared with the control group. We began a controlled clinical trial with modern treatment (including transplant procedures) in combination with zoledronic acid to define the role of zoledonic acid in this setting of patients.