Anterior cruciate ligament (ACL) reconstruction depends critically on isometric graft placement. Unfortunately, different supposedly isometric points have been published, and no prior work has compared them to find out which are really isometric. The purpose of this study was to compare the isometry of previously published 'isometric' points for ACL reconstruction. The isometric points and knee loadings of previous studies were reproduced accurately in 12 fresh cadaveric knees. The length changes were measured through 140 degrees knee flexion, using an intra-articular suture attached to a displacement transducer. Six points had less than 1 mm length change and were located proximally in the natural ACL attachment at the posterior end of Blumensaat's line. The other seven points had length change patterns that would cause ACL graft tightening or slackening with knee flexion if they were used as the sites of bone tunnels for graft placement. This study confirms the existence of an isometric zone close to the posterior end of Blumensaat's line under several loading conditions. Other graft attachment points are less suitable for ACL reconstruction.