INTRODUCTION: Osteonecrosis of the jaw (ONJ) has been reported in patients treated with bisphosphonates. The incidence and risk factors associated with this disorder have not been clearly defined. MATERIALS AND METHODS: We conducted a retrospective analysis of 4019 patients treated with intravenous bisphosphonates between 1996 and 2004. Our goals were to estimate the frequency, understand the clinical presentation, and identify risk factors associated with ONJ development. RESULTS: Sixteen of 1338 patients with breast cancer (1.2%) and 13 of 548 patients with multiple myeloma (2.4%) developed ONJ. The median dose and duration of treatment with pamidronate or zoledronic acid were significantly higher in patients with ONJ (p