The purpose of this study was to evaluate the results of prosthetic ligament replacement of the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) with the Gore-Tex polytetrafluorethyene prosthesis (W.L. Gore and Co., Flagstaff, Ariz. ) in 52 patients (54 knees). All patients sustained multiple (failed) knee operations or had knees with gross instability. Twenty-eight (29 knees) of the 52 patients (54%) in whom the Goretex prosthesis was still in situ were available at a minimum follow-up of 5 years (mean 9 years, range 5-11 years). The mean age at examination was 39 years (range 30-57 years); there were 15 men and 13 women. The results of the procedure were compared with the results of the same patients at a mean follow-up of 3 years. Eighty-one percent of the patients of the whole group complained about pain. This was 78% for the patients with an ACL reconstruction and 75% for the patients with a PCL reconstruction. The Tegner activity score and the Lysholm knee score showed a statistically significant difference over time. The anterior instability pattern improved in only 43% of the knees and the posterior instability in 41% of the knees. The Lachman test showed also a significant difference over time. In all patients X-ray showed an increase in degenerative changes. In conclusion, the PTFE prosthetic ligament in the reconstruction of the ACL and the PCL in the (chronically) unstable knee seems to deteriorate over time.