Abstract Several non-invasive techniques have been developed to assess anterior–posterior (A–P) laxity of the knee, however, their accuracy remains unclear. Roentgen Stereophotogrammetry Analysis (RSA) is a three-dimensional motion analysis method that has been shown to be an accurate tool for evaluating joint kinematics. Thus, RSA provides a means that can be used to evaluate other less invasive techniques. The objectives of this study were to compare A–P laxity values as measured using the KT-1000 Knee Arthrometer, planar stress radiography and RSA, and to determine if they detect similar changes in A–P laxity over time in 15 subjects following anterior cruciate ligament reconstruction with patellar tendon autografts. The A–P laxity values of the injured knee were measured immediately following surgery and at their 3-, 6-, and 12-month follow-up visits. A–P laxity was defined as the A–P translation of the tibia relative to the femur that occurred between the posterior and anterior shear load limits of −90 to +130 N, respectively. The values of A–P laxity across all time points were 11.4±3.0, 10.2±3.3, and 6.9±3.0 mm (mean ± standard deviation) for the KT-1000, planar stress radiography and the RSA methods, respectively. These values were significantly different from each other ( p<0.001). The two-dimensional analyses techniques (planar stress radiography and the KT-1000) consistently over-estimated the true laxity values that were obtained using the three-dimensional RSA technique. Significant increases in A–P laxity values over time were also detected with the KT-1000 ( p=0.04) and the RSA technique ( p=0.04). However, this increase was not evident when using planar stress radiography ( p=0.89). This study determined that the KT-1000 and RSA document temporal changes in A–P laxity following ACL reconstruction that were not documented by planar stress radiography.