OBJECTIVE: To test the hypothesis that bone mineral density (BMD) in periarticular subchondral regions of the knee joint is abnormal in patients with osteoarthritis (OA). METHODS: Sixty-two knees from patients with relatively mild OA (primarily radiographic grades 0, 1, 2) of the knee were compared with 62 knees from normal subjects matched for age, sex, race, and body side (right or left). BMD of the lumbar spine, distal femoral shaft, and several periarticular subchondral regions of the knee were determined by dual energy x-ray absorptiometry (DXA). To facilitate increased accuracy of BMD values, lateral DXA of the knees was also performed and used to provide a third dimension to the usual two-dimensional anterior-posterior g/cm2, producing measurements as g/cm3. Subchondral bone regions included both superficial and deep regions of the medial femoral, medial tibial, and lateral tibial compartments. RESULTS: BMD (g/cm3) was lower than normal in 6 subchondral bone regions of the knees (n=43) of white female patients with OA (average decrease -13.3%), significant in all 6 subchondral regions (p=0.001 to p=0.047). Two-dimensional BMD (g/cm2) was lower than normal in 6 subchondral regions of the knees (n=43) of white female patients with OA (average decrease -7.3%), significant in only 2 of 6 subchondral regions (p=0.011 to p=0.014). BMD (g/cm3) was lower than normal in 6 subchondral regions of the knees (n=19) of African American female patients with OA (average decrease -9.1%), significant in only one of the 6 subchondral regions (p=0.016). There was no significant difference in spinal BMD (L1-L4) or femoral shaft BMD between normal and OA for either racial group. About 13% of the OA patients had osteoporosis at the spine. CONCLUSION: A significant decrease in periarticular subchondral BMD is present in female patients with relatively mild OA of the knee whether or not they had osteoporosis based on a spine BMD measurement.