In this prospective study we investigated 70 patients with chronic anterior instability who underwent anterior cruciate ligament reconstruction with a Dacron prosthesis. Fifty-five (79%) patients could be followed up at a mean of 9 years. Within the follow-up time, 44% (29 of 66) of the prostheses were diagnosed as ruptured; the annual rupture frequency was 5%. Second reconstructions were performed for 20 patients (29%), 5 patients had third reconstructions, and 9 patients wanted no further surgical treatment. Fifty-two patients (74% of the original group) underwent radiographic examination. Eighty-three percent (43) of these patients had radiographic changes in the operated knee, and 40% (21 of 52) had joint space reduction up to 50%. The rate of radiographically detected osteoarthritis was increased 10 times compared with the normal knee. Patients with medical instability and patients older than 30 years at the initial operation had more severe osteoarthritic changes. Based on the functional results of the patients with a ligament in place after 9 years, only 14% (10) of the original group had acceptable stability and knee function.