Stiffness of the medial (MCL) and lateral (LCL) collateral ligaments was compared between a group of 10 patients undergoing total knee arthroplasty for varus degenerative osteoarthritis (OAP), a group of 10 osteoarthritic cadaveric knees (OAC), and a group of 10 non-osteoarthritic cadaveric knees (NOA). A load-elongation curve was obtained for the medial and lateral compartments of each knee using an instrumented Moreland spreader. A strain gage (SG) was attached to the spreader handle and strain was calibrated to load applied against the spread distance. In extension, medial compartment stiffness of the OAP, OAC, and NOA groups was 60.7 ± 16, 52.8 ± 9.3 and 21.4 ± 5.0 N/mm, respectively. Lateral compartment stiffness in extension was 29.2 ± 9.2, 33.3 ± 10.3 and 19.5 ± 5.3 N/mm, for OAP, OAC, and NOA, respectively. Differences in stiffness between the OAP and OAC groups were not statistically significant (p > 0.05). However, the osteoarthritic groups (OAP and OAC) demonstrated a statistically significantly (p < 0.05) increase in ligament stiffness when compared to the NOA group. Following knee arthroplasty, stiffer medial structures in extension may lead to flexion contracture and accelerated polyethylene wear. Adequate bone resection, in conjunction with soft tissue release may alleviate the threefold increase in stiffness observed in the medial compartment secondary to OA without jeopardizing joint stability. © 2002 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.