This clinical study evaluates the minimum 6 year follow-up clinical, functional, radiological and isometric results after arthroscopic reconstruction of the anterior cruciate ligament (ACL) with quadrupled semitendinosus tendon autograft (femoral endobutton and tibial suture disc fixation). Seventy out of 85 operated patients (lost to follow-up 17%) with an average age of 34.3 years could be clinically examined at a mean follow-up time of 6 years and 4 months after surgery. Results revealed a high subjective satisfaction rate of 93%. The preoperative activity level could be maintained in 71% of the patients. The Lysholm score showed very good and good results in 85% with a mean of 83.6%. Normal or nearly normal results on the IKDC score were reported in 85% of the patients. The KT-1000 arthrometer stability testing showed a difference of less than 3 mm compared to the contralateral knee in 75% of the patients. Five patients (7%) had a graft failure during follow-up time. There were no or only mild degenerative changes at the radiographic evaluation in 85% of the patients. In 15% of the patients a femoral and in 40% a tibial tunnel widening of more than 50% was observed, however without a relevance for the clinical and functional status. Midterm results obtained in this study after arthroscopic ACL reconstruction with the quadrupled semitendinosus tendon confirm the outcomes in the literature after shorter follow-up periods that provide very good and good subjective, functional and stability results in about 80-85% of the patients. This surgical technique can be recommended for the active patient with ACL deficiency. However, patients must be informed that activity level cannot always be maintained and a failure rate of 5-10% must be taken into account in the longer term when decision for surgery is made.