ACL (anterior cruciate ligament) partial tears include various types of lesions, and an high rate of these lesions evolve into complete tears. Most of the techniques described in literature for the surgical treatment of chronic partial ACL tears, don't spare the intact portion of the ligament. Aim of this study was to perform a prospective analysis of the results obtained by augmentation surgery using gracilis and semitendinosus tendons to treat partial sub-acute lesions of the ACL. This technique requires an “over the top” femoral passage, which enables salvage and strengthening of the intact bundle of ACL. The study included 97 patients treated consecutively at our Institute from 1993 to 2004 with a mean injury-surgery interval of 23 weeks (12–39). Patients were followed up by clinical and instrumental assessment criteria at 3 months, 1 year and 5 years after surgery. Clinical assessment was performed with the IKDC form. Subjective and functional parameters were assessed by the Tegner activity scale. Instrumental evaluation was done using the KT-2000 instrument: the 30 pound passive test and the manual maximum displacement test were performed. We obtained good to excellent results in 95.9% of cases. We didn't observed recurrences in ligamentous laxity. We believe that the described technique has the advantage of being little invasive, compatible with the ACL anatomy, and enables very rapid functional recovery and return to sport.