Sixty-four subjects having implanted and nonimplanted knees were studied using fluoroscopic videos. Each subject, flexing in the sagittal plane, performed successive deep knee bends under fluoroscopic surveillance. Femorotibial contact in the sagittal plane was then determined using image matching and discrete digitization. At full extension, the mean contact point of the normal and posterior stabilized implanted femurs was anterior to the tibial midpoint in the sagittal plane. The average position was 6.49 mm (+3 - +13 mm) for the normal knees and 0.30 mm (0 - +4 mm) for the posterior stabilized knees. The implanted posterior cruciate retaining and anterior cruciate ligament deficient knees differed from the other knee types. Their average initial contact was posterior. The average contact at full extension for the posterior cruciate retaining and anterior cruciate ligament deficient knees was -5.13 mm (-2 - -8 mm) and -5.45 mm (-2 - -14 mm), respectively. The femur of the normal knee contacts the tibia anterior to the midpoint in the sagittal plane in full extension and translates posteriorly during flexion. The femur of the posterior stabilized knee contacts the tibia anteriorly, slightly less than the normal knee, and rolls back posteriorly during flexion similar to normal knees. The femurs of the posterior cruciate retaining and anterior cruciate ligament deficient knees contact the tibia posterior in extension, but translate anteriorly during midflexion in a substantial number of cases, which is kinematically opposite of the normal knees. The abnormal anterior femoral translation observed in the posterior cruciate retaining knees may be a factor in the premature polyethylene wear seen in retrieval studies.