Introduction and Objective.A common belief is that after an anterior cruciate ligament (ACL) injury, there is a deficit knee range of motion (ROM). Deficit ROM can be caused by swelling, postoperative or postinjury pain. However, in the literature there is a lack of papers proving that there is a deficit of knee joint extension ROM after ACL injury. The aim of the study was to compare the knee joint extension range between the healthy limb and the limb with an anterior cruciate ligament (ACL) injury. Materials and Method. The study was performed on a group of 44 patients aged 18-46 years with ACL injury (non-operative). The diagnosis was made on the basis of functional tests: Lachman test, pivot shift test, anterior drawer test, confirmed by MRI examination. ACL damage was also diagnosed in the MRI report by a radiologist. A Saunders inclinometer was used to measure passive and active knee extension.Results. There was a significant difference in the measurements of knee extension between a healthy limb and a limb with an ACL injury (active extension p=0.0012; passive extension p=0.0122). Conclusions. The limb with ACL injury had a lower range of extension in comparison to the healthy limb. Therefore, treatment focusing on improving the range of extension seems to be beneficial in patients' recovery. It is important to examine both the active and passive knee extension range of motion after ACL damaged.