We performed stress arthrometric studies on 77 knees (71 patients) with total knee arthroplasty to determine the presence and magnitude of femoral abduction and adduction in knee extension. A total of 53 knees (49 patients) had posterior cruciate ligament-retaining (PCLR) prostheses, and 24 (22 patients) had PCL-substituting (PCLS) prostheses. The selected patients had successful arthroplasties with no clinical complications a minimum of 5 years after primary surgery. Each patient was subjected to a successive abduction and adduction stress test at 0 degrees -20 degrees of flexion using a Telos arthrometer. The mean values for abduction and adduction were 4.8 degrees and 4.5 degrees with a PCLR prosthesis, respectively, and 4.6 degrees and 4.0 degrees with a PCLS prosthesis. There were no statistical differences between PCLR and PCLS knees. The results suggest that approximately 4 degrees of laxity in these directions is suitable in total knee arthroplasty for a satisfactory clinical outcome 5-9 years after surgery.