To evaluate the clinical outcome of arthroscopic treatment of ACL with an Achilles tendon allograft in patient with acute rupture. 22 patients, between 2003 and 2006, with acute rupture of ACL, were treated with an Achilles tendon allograft. The mean age was 26 years. Patients were evaluated before and after surgery and at the latest follow-up with Noulis-Lahmann test and Pivot shift test. We also used IKDC score, Lysholm score and one leg stance test and functional reach test. Patients were also evaluated with Cybex II + and with plain radiographies. The mean follow-up time was 3.5 years. 90% of the patients had a negative pivot shift test and 95% of the patients had a score at Noulis-Lahmann test +1. The mean value of IKDC score was 88 (62–100) and the mean time of Lysholm score was 91 (75–100). Until the latest follow-up there were no clinical sighs of inflammation or graft rejection. Radiologic evaluation revealed no sign of tunnel enlargement. We believe that the use of a fresh-frozen allograft in the treatment of acute ACL ruptures is an effective procedure for the restoration of ligamentous stability of the knee.