The purpose of this study is to clarify the definition, clinical assessment, and strategies for treatment of partial tears of the anterior cruciate ligament. Both the diagnosis and natural history of partial tears of the anterior cruciate ligament are controversial issues. A partial tear of the anterior cruciate ligament is characterized by an asymmetric Trillat- Lachman-test result, a negative pivot-shift test, a low-grade KT-1000 arthrometer measurement (< or = 3 mm), and arthroscopic evidence of anterior cruciate ligament injury. The pivot-shift test is the most important test in determining anterior cruciate ligament insufficiency. A positive test, independent of the grade, is indicative of a functionally deficient anterior cruciate ligament. Nonoperative management results in an acceptable clinical outcome in the majority of cases. Progression to knee instability (anterior translation and rotation) depends on the extent of the anterior cruciate ligament injury and the activity level of the patient. Thermal shrinkage is not recommended for treatment of partial anterior cruciate ligament tears. Symptomatic instability is treated reliably with anterior cruciate ligament reconstruction. A partial tear of the anterior cruciate ligament should be defined functionally and not structurally, to guide subsequent treatment.