This study looks at the difference between the macroscopic and microscopic appearances of the anterior cruciate ligament (ACL) in a sample of 55 consecutive patients admitted for routine total knee replacement for osteoarthritis. At the time of surgery the macroscopic appearance of the ACL was classified as normal, moderately damaged (fissured) or completely ruptured. The excised ACL was sent for histological examination and grading. The macroscopic appearance of the ACL at surgery was compared to the severity of disease on microscopic examination. At surgery, 31 ACLs were found to be macroscopically normal: 22 of these (71%) showed moderate to severe disease on microscopic assessment. Thus a macroscopically normal ACL does not necessarily imply histological integrity. This has clinical implications in other areas of knee surgery including Unicompartmental Knee Replacement which require a fully functional intact ACL.