Purpose: The aim of this 5-year longitudinal study was to investigate the influence of the retention mechanism for overdenture fixation on the periimplant parameters of mandibular implants. Materials and Methods: Ninety edentulous patients, each with two interforaminal implants supporting an overdenture, participated in the study. They had received either a straight or slightly bent round clip bar, a U-shaped rigid bar with or without distal extensions, or single-ball anchors. Anatomic-morphologic and prosthetic variables were assessed related to the implant position and the type of overdenture retention. The degree of mandibular atrophy was determined using cephalometric radiographs. The angle β between the virtual axis connecting both implants and the mandibular hinge axis was measured on mounted casts. A supporting surface was identified between bent clip bars and U-shaped extension bars. Clinical periimplant parameters were recorded, and changes of the probing attachment level after 5 years were analyzed in 70 patients with respect to the retention device and anatomic-morphologic and prosthetic variables. Results: A significant increase of the Plaque Index was observed after 5 years. The mean clinical attachment loss after 5 years was about 0.2 mm. In more than 50%, β was small, ie, ¡Ü 5 degrees. For these implants, loss of clinical attachment was significantly lower. The correlation between attachment loss and β was weak, while no significant correlation was found with regard to the degree of atrophy and surface. Conclusion: In view of the long observation period, it was concluded that the position and retention mechanism of mandibular implants supporting an overdenture have little influence on the long-term stability of the clinical parameters. Int J Prosthodont 2001;14:536¨C542.