Osteonecrosis of the jaw (ONJ) has recently been reported as a potentially serious complication of prolonged treatment with intravenous bisphosphonates. We studied its frequency in prostate cancer patients receiving intravenous zoledronate. The medical and dental records of 52 consecutive patients with prostate cancer and bone metastases treated at our institute between January 2002 and October 2005 were reviewed. All patients received intravenous zoledronate 4 mg every 3 or 4 weeks and concomitant conventional prostate cancer treatments. We analysed the association of ONJ with the number of administrations of zoledronate and exposure to chemotherapy. At a median follow-up of 7 months (range 1-41) after the initiation of zoledronate, ONJ occurred in six patients (12%, 95% C.I. 5.4-23.0%). All six ONJ cases occurred after the 9(th) administration of zoledronate. The median number of zoledronate administrations was 17 (range 9-24) and 8 (range 1-32) for patient developing and not developing ONJ, respectively (p =0.02). Chemotherapy with docetaxel was also associated with a strong, but not statistically significant, trend towards increased risk of ONJ (OR 3.8, 95% C.I. 0.4-35.6, p =0.24). The length of exposure to zoledronate was associated with an increased frequency of ONJ in prostate cancer patients. A possible role of chemotherapy with docetaxel as a cofactor for ONJ merits further evaluation.