The rate of change in osteoarthritic (OA) tibial articular cartilage and the factors that influence it are not known. We examined a cohort of subjects with OA to determine the change in articular knee cartilage volume over the course of 2 years and to identify factors which might influence such change and its rate. One hundred twenty-three subjects with OA underwent baseline knee radiography and magnetic resonance imaging (MRI) on their symptomatic knee. They were followed up 2 years later with a repeat MRI of the same knee. Knee cartilage volume was measured at baseline and at followup. Risk factors assessed at baseline were tested for their association with change in knee cartilage volume over time. Mean +/- SD total tibial articular cartilage decreased by 5.3 +/- 5.2% (95% confidence interval [95% CI] 4.4%, 6.2%) per year. The annual percentages of loss of medial and lateral tibial cartilage were 4.7 +/- 6.5% (95% CI 3.6%, 5.9%) and 5.3 +/- 7.2% (95% CI 4.1%, 6.6%), respectively. Initial cartilage volume was the most significant determinant of loss of tibial cartilage in all compartments, while age was a significant determinant of lateral tibial cartilage loss, when possible confounders were accounted for. In OA, tibial cartilage volume is lost at a rate of approximately 5% per year. The main factor affecting cartilage loss is initial cartilage volume. Our results suggest that cartilage loss may be more rapid early in disease. Further study is required to determine whether the rate of cartilage loss in OA is steady or phasic, and to identify factors amenable to intervention to reduce cartilage loss.