OBJECTIVES:   Clinical case reports support the occurrence of atypical fractures while Phase III trials for bisphosphonates have not shown this adverse effect.  This study determines whether or not atypical fractures of the subtrochanteric region and diaphyseal femur are occurring in bisphosphonate users within the Kentucky Medicaid population. The incidence of atypical femur fractures in bisphosphonate users has been estimated at 30 per 100,000 person years.1  Bisphosphonates are commonly prescribed for the prevention of osteoporosis-related fractures,2  but there are concerns about this possible, but rare, adverse event related to the long-term use of bisphosphonates.3,4 METHODS:   The retrospective Kentucky Medicaid claims based study assesses the risk of atypical fracture in bisphosphonate treated patients versus non-bisphosphonate treated patients, who experienced a baseline fracture.  Of the 19,988 patients, aged 45-95, 2,440 of them used a bisphosphonate for at least 180 days.  RESULTS:   There were 209 patients experiencing an atypical fracture.  Baseline fractures were grouped and those experiencing atypical fractures versus those who didn’t are as follows: humerus (1.95% vs 0.97%), lower extremity (3.04% vs 0.97%) and vertebral (1.58% vs 0.95%). Unadjusted analysis indicated that bisphosphonate users are two times more likely to have atypical fracture (OR: 2.2271 95%CI: 1.6123-3.0758).  Adjusting for demographics, baseline fracture, Charlson comorbidity index, and use of corticosteroids the patients who used a bisphosphonate for at least 6 months were 1.92 times more likely to have atypical fracture than non-users (95%CI: 1.365-2.688).  Age was a significant predictor in the multivariate model; bisphosphonate users, 65 and older, were three times more likely to have an atypical fracture (OR: 3.187 pvalue<0.001). CONCLUSIONS:   While a relationship between bisphosphonate use and atypical fractures is present, the preventative function of other fractures dictates continued therapy.  However, more research is needed to determine if a certain subtype of patients can be identified to prevent future atypical fractures.