Preliminary studies have shown that dual-energy X-ray absorptiometry (DXA) produces images of sufficient quality for a precise and accurate measurement at density of the subchondral bone. The objective of this study was to investigate the relationship between baseline subchondral tibial bone mineral density (BMD) and joint space narrowing observed after 1 year at the medial femoro-tibial compartment of the knee joint. Fifty-six consecutive patients, from both genders, with knee osteoarthritis diagnosed according to the American College of Rheumatology criteria, were included in the study. Radiographic posteroanterior views were taken, at baseline and after 1 year of follow-up. Minimum joint space width (JSW) measurement, at the medial femoro-tibial joint, was performed with a 0.1-mm graduated magnifying lens. Baseline BMD of the subchondral tibial bone was assessed by DXA. The mean +/- SD age of the patients was 65.3 +/- 8.7 years, with a body mass index of 28.0 +/- 4.9 kg/m(2). The minimum JSW was 3.5 +/- 1.5 mm and the mean BMD of the subchondral bone was 0.848 +/- 0.173 g/cm(2). There was a significant negative correlation between subchondral BMD and 1-year changes in minimum JSW (r = -0.43, p = 0.02). When performing a multiple regression analysis with age, sex, body mass index, and minimum JSW at baseline as concomitant variables, BMD of the subchondral bone as well as JSW at baseline were independent predictors of 1-year changes in JSW (p = 0.02 and p = 0.005, respectively). Patients in the lowest quartile of baseline BMD (<0.73 g/cm(2)) experienced less joint space narrowing than those in the highest BMD quartile (>0.96 g/cm(2)) (+0.61 +/- 0.69 mm versus -0.13 +/- 0.27 mm; p = 0.03). Assessment of BMD of the subchondral tibial bone is significantly correlated with future joint space narrowing and could be used as a predictor of knee osteoarthritis progression.