Few studies have examined the effect of primary total knee arthroplasty on the weight and physical activity of overweight and obese patients in the United States. We conducted a prospective study of changes in mean weight, body mass index (BMI), and physical activity over 2 years in 188 consecutive overweight or obese patients. Weight, BMI, and physical activity, evaluated using the Lower Extremity Activity Scale (LEAS), were assessed preoperatively and at 1 and 2 years. At 2 years, no significant weight change was found (P = .80), but BMI increased by 0.46 kg/m 2 (P = .049). The LEAS score increased from preoperatively to 2 years (P < .001). Preoperative LEAS score was not associated with weight or BMI at 2 years. This finding has implications for patient expectations and preoperative counseling.