In a review of the recent literature after conservative treatment of the complete anterior cruciate ligament (acl) rupture good results were found in an average (av) of 56% (12-80%) of all cases. In comparison the conservative treatment of partial tears showed in 84% (60-100%) of all cases good results. 79% (32-100%) of the patients with complete acl ruptures and 90% (66-100%) of the cases with partial acl tears were able to continue their sport activities. Secondary acl replacement or meniscus surgery were later necessary in 20% and 14% of the cases (av) with complete acl ruptures and in only 6% and 5% (av) of the cases with partial tears. Most of the patients who were operated later were young and ambitiously engaged in sports. The results of the mostly retrospective studies display a high variability. For this reason it is necessary to perform more prospective, controlled and randomized studies to verify the preventive effect of an early operative treatment to protect the joint form osteoarthritis and progressive instability. Arthrocopy is essential for all cases of acute hemartrosis. Partial acl tears are predominantly treated conservatively, while complete acl ruptures should be treated following an individual decision regimen. A tree for decision making of the treatment of acute acl-injuries is proposed.