The in vivo kinematics of two types of unconstrained, posterior cruciate ligament retaining knee prostheses were analyzed 1 year postoperatively using roentgen stereophotogrammetric analysis. Ten knees had the Miller-Galante and five the New Jersey LCS design. The Miller-Galante knees displayed decreased adduction during active flexion when compared with normal knees. In both types of prostheses, there was decreased medial and increased proximal and posterior displacement. The abnormal kinematics probably reflect the design of the articular surfaces, the absence of the anterior cruciate ligament, and the dysfunction of the posterior cruciate ligament.