The incidence of osteonecrosis of the jaw (ONJ) in the population is low, but specifics are unknown. Potential risk factors include bisphosphonate treatment, steroid treatment, osteoporosis, and head/neck radiation. This Dental Practice-Based Research Network study estimated ONJ incidence and odds ratios from bisphosphonate exposure and other risk factors using a key word search and manual chart reviews of electronic records for adults aged > or = 35 years enrolled during 1995-2006 in 2 large health care organizations. We found 16 ONJ cases among 572,606 cohort members; 7 additional cases were identified through dental plan resources. Among 23 cases (0.63 per 100,000 patient years), 20 (87%) had at least 1 risk factor, and 6 (26%) had received oral bisphosphonates. Patients with oral bisphosphonates were 15.5 (CI, 6.0-38.7) more likely to have ONJ than non-exposed patients; however, the sparse number of ONJ cases limits firm conclusions and suggests that the absolute risks for ONJ from oral bisphosphonates is low.