There four indications for arthroscopy in patients with ruptures of the anterior cruciate ligament: (1) assessment of the acutely injured knee, which will usually have a hemarthrosis; (2) assessment of the knee with late symptoms following ligament injury; (3) a preliminary to ligament reconstruction to assess the integrity of the menisci; (4) the assessment of patients who have symptoms following ligament reconstruction. Many patients achieve a good result after arthroscopic correction of meniscal lesions and other derangements without ligament reconstruction. Repair of all acutely ruptured anterior cruciate ligaments will result in many unnecessary ligament repairs. Assessment of acute ligament repairs is made difficult by the absence of a controlled series. The criteria for distinguishing those patients who need ligament reconstruction from those who will do well with aggressive conservative management alone have yet to be established. A prosthetic ligament can be inserted under arthroscopic control without arthrotomy, but no suitable prosthetic material is yet available.