We did an in vivo fluoroscopic study comparing the sagittal plane kinematics of mobile-bearing and fixed-bearing total knee arthroplasties in a unique group of patients. These patients were part of a larger bilateral randomized controlled outcome trial with each patient having received both types of total knee arthroplasties. Invited patients did three exercises with each of their different knee replacements; extension against gravity, flexion against gravity, and a step-up. These exercises were recorded using video fluoroscopy, and a series of still digital images over the flexion range were retrieved. The relationship of patella tendon angle to knee flexion angle for each patient was derived. The patella tendon angle to knee flexion angle of the mobile-bearing knee behaved in a linear manner more closely replicating the normal knee, whereas the fixed-bearing knee behaved in a nonlinear, more variable manner. This pattern of results was similar for all three exercises with each patient having one knee replacement that behaved differently in the sagittal plane when compared with their other knee replacement. These kinematic differences may explain the clinical differences observed in the randomized controlled trials that compared these two total knee arthroplasties.