Objective: There is evidence for tibial bone area to increase in response to risk factors for knee osteoarthritis (OA) in healthy subjects and to increase over time in subjects with knee OA. We performed a cohort study to examine whether tibial plateau bone area changes over time in healthy subjects and identify factors influencing the change. Design: Eighty-one healthy women (age range 50-76 years) underwent magnetic resonance imaging (MRI) on their dominant knee at baseline and approximately 2.5 years later. Tibial plateau bone area was measured at baseline and follow-up. Risk factors assessed at baseline were tested for their association with change in tibial plateau bone area over time using multiple linear regression. Results: The mean tibial plateau bone area increased from 1733 +/- 209 to 1782 +/- 203 mm(2) for the medial, and from 1090 152 to 1109 +/- 152 mm(2) for the lateral over the study period, representing an annual average increase rate of 1.2 (95 Cl 0.03 , 1.6 ) and 0.8 (95 Cl 0.7 , 1.8 ), respectively. Baseline tibial plateau bone area was inversely associated with the increase rate of tibial plateau bone area. There was a trend for static knee alignment to be related to the increase rate of tibial plateau bone area. Conclusion: In healthy women, tibial plateau bone area increases over time. Baseline tibial plateau bone area is the main factor affecting the rate of increase, with biomechanical factors, such as static anatomical alignment, likely to affect the expansion of tibial plateau. Further work will be needed to determine the effect of subchondral bone change in the pathogenesis of knee OA. (C) 2006 OsteoArthritis Research Society International. Published by Elsevier Ltd. All rights reserved.