Objective: Since 2003, multiple cases of bisphosphonate-induced osteonecrosis of the jaw (ONJ) were reported. The aim of this study was to describe the incidence and risk factors of ONJ in patients with breast cancer or gynecological malignancies receiving bisphosphonates (BP). Methods: ONJ was recorded for all patients with breast cancer or gynecological malignancies treated with intravenous bisphosphonates at the Department of Gynecology and Obstetrics, University Hospital Tuebingen during April, 1999 and May, 2006. Results: 10 of 345 (2.9%) patients with breast cancer or gynecological malignancies developed ONJ while receiving bisphosphonate therapy. Six patients with ONJ had a history of recent dental procedures. All patients had received zoledronic acid as part of their bisphosphonate regimen. Time of exposure to bisphosphonates and the number of treatment cycles were significant risk factors for the development of ONJ (p < 0.001). In patients diagnosed with ONJ the mean number of treatment cycles was 27 ± 18 cycles. However, the mean number of treatment cycles in patients without manifestation of ONJ was 12 ± 12 cycles. Conclusion: Length of exposure to BPs and the cumulative dose of given BPs seem to be the most important risk factors for the development of ONJ followed by dental procedures.