A series of 51 patients (55 knees) with implanted GUEPAR totally constrained endoprosthesis was followed up clinically and radiologically for a mean of 5.0 years (range 1.5-11.8 years). Good relief of pain, increased range of knee movement and of walking capability were observed. There was, however, an unacceptable amount of severe complications. Three endoprostheses had to be removed and arthrodesis performed. Revision arthroplasty was carried out in 8 cases due to aseptic loosening of the prosthesis. In 25 cases lateral displacement or total dislocation of the patella was confirmed with tangential patellar X-ray projections. We suggest that the GUEPAR totally constrained total knee endoprosthesis and similar constructions should be chosen in only special cases, when aged patients have extremely damaged and unstable knee joints and non-constrained prostheses are not considered suitable.