Purpose: This prospective investigation studied the clinical and radiographic performance of mandibular fixed prostheses supported by osseointegrated implants over more than 20 years. Materials and Methods: A total of 273 standard Brånemark implants (10 mm long) were placed in 47 patients between 1978 and 1982. Clinical and radiographic data collected at several examinations over the 20-year observation period have been reported previously. This study presents the outcome of the latest follow-up after 20 to 23 years. Results: Thirty patients (64%; 75% of those still alive) attended the 20-year follow-up examination. Three implants were lost during the entire observation period, and the 20-year implant cumulative survival rate was 98.9%. All patients had continuous prosthesis function, but two had their mandibular prostheses remade during the 20 years. No implants or prostheses were lost or fractured during the last 5 years, and only a few prosthodontic complications were noted. The mean bone level was 1.6 mm (SD 0.90) below the reference point after 20 years, and mean bone loss was 0.2 mm (SD 0.22) between the 15- and 20-year follow-ups. Thirtyseven implants (24%) showed more than two exposed threads at the 15-year follow-up examination, but only four implants (3%) presented pain and/or bone loss exceeding one thread (0.6 mm) during the last 5 years. Conclusion: The successful treatment result after 15 years continued up to more than 20 years in function. During the last 5 years, a majority of the implants with several exposed implant threads could be maintained without any complications, and the frequency of implants showing signs of ongoing peri-implantitis was less than 3%. Int J Prosthodont 2003;16:602–608.