Patients who have osteoarthrosis involving weight-bearing joints typically have a gradual decline in physical activity, which is often associated with cardiovascular deconditioning. After joint replacement, many patients resume routine walking and recreational activities, which may improve aerobic capacity. The purpose of the present study was to determine the effect of total knee arthroplasty on cardiovascular fitness.Nineteen patients who had had total knee arthroplasty (the arthroplasty group) performed an exercise test for cardiovascular fitness preoperatively. Sixteen of these patients were tested again at one year postoperatively and thirteen, at two years postoperatively. Sixteen patients in whom osteoarthrosis of the knee was being treated medically (the control group) were tested at the time of enrollment in the study and one year later. Nine of these patients also were tested two years after enrollment. All of the patients completed the Arthritis Impact-Measurement Scales (AIMS) at each examination.Fitness was assessed by the performance of a progressive maximum exercise test with use of a bicycle ergometer and a metabolic cart. All patients achieved the anaerobic threshold. One year postoperatively, the patients in the arthroplasty group demonstrated an increase, which approached significance, in maximum oxygen consumption (measured in milliliters of oxygen per minute) (p = 0.07), maximum oxygen consumption corrected for body weight (p = 0.08), and percentage of predicted maximum uptake of oxygen (p = 0.06). Two years after the total knee arthroplasty, the patients had a significant improvement with regard to all three parameters (p = 0.008, 0.005, and 0.005, respectively). The patients in the control group demonstrated a significant decrease in duration of exercise and in maximum workload at one year (p = 0.003 and 0.005, respectively) and at two years (p = 0.008 for both parameters). Physical activity had increased in the arthroplasty group but not in the control group, as demonstrated by the results of the Arthritis Impact-Measurement Scales.These findings demonstrate a trend toward improvement in cardiovascular fitness one year after total knee arthroplasty and a significant improvement two years postoperatively for patients who had been able to resume routine functional activities because of the arthroplasty. These improvements compared favorably with the static pattern or the decline in the measures of fitness that were observed in the control group. Our results should be considered preliminary because of the relatively small number of patients who were studied.