A stress arthrometric study was done on 60 knees in 54 patients with total knee arthroplasties using a Telos arthrometer, to determine anteroposterior and abduction and adduction laxity and to evaluate the relationship between laxity and retention of the posterior cruciate ligament using low contact stress mobile-bearing prostheses. Thirty knees had posterior cruciate ligament-retaining and 30 had posterior cruciate ligament-sacrificing prostheses. The selected patients had successful knee arthroplasty 6 months previously. Anteroposterior displacement was measured at 30 degree and 90 degree flexion; there were no statistically significant differences between the posterior cruciate ligament-retaining (10.5 mm at 30 degree, 9.3 mm at 90 degree) and posterior cuciate ligament-sacrificing (9.8 mm at 30 degree, 9.7 mm at 90 degree) groups. Abduction and adduction were between 0 degree and 20 degree flexion, there were no significant differences between the two groups. Because all the patients in this study had good clinical results, approximately 10 mm anteroposterior displacement and 4 degree laxity in the coronal direction are considered favorable in low contact stress mobile-bearing prostheses of both designs.