Arthroscopy for diagnosis in isolated rupture of the cruciate ligaments of the knee. Arthroscopy of the knee was performed in the years 1981 through 1983 in 1107 patients, arthroscopic meniscectomy in 340 patients. In acute traumatic haemarthrosis of the knee 74 "isolated" ruptures of the cruciate ligaments were found. The exact disclosure of these 74 cases gave a real isolated rupture of the anterior cruciate in 28.4%, an isolated rupture combined with meniscus tears and connected with flake fracture each in 20.3%. In 17.5% the exact diagnosis was a disruption of the anterior cruciate together with meniscus tears and flake fracture. Acute arthroscopy allows a more accurate diagnosis of injury to joint structures. If there was an intraligamentary rupture of the anterior cruciate and meniscus tear or flake fracture we did arthroscopic meniscectomy and removed the small flake. The substitution of the anterior cruciate was performed, if there were instability complains, later on at agreeable time for the patient.