Purpose: This retrospective study sought to examine posterior mandibular ridge resorption under implant-supported and implant-retained distal extension partial overdentures in men at the end of a 5-year observation period. Materials and Methods: Class I mandibular partial edentulism was managed in 34 patients with removable partial overdentures that were adjunctively supported (n = 18) or retained (n = 16) via resilient attachments placed bilaterally on single implants (n = 68) in the first molar areas. Posterior Area Indices (PAI) were calculated for each patient by digitizing the traced rotational tomograms taken immediately before and after 5 years of treatment. Proportional rather than actual measurements were used in an effort to minimize errors related to magnification and distortion. Results: Residual ridge resorption associated with the implant-supported partial overdentures was recorded as PAI =0.012 ± 0.022; it was PAI = 0.073 ± 0.044 for the implant-retained group. Estimated average reductions in ridge heights were 0.15 and 1.03 mm for implant-supported and implant-retained partial overdentures, respectively. Multiple linear regression models demonstrated that prosthesis type, initial mandibular ridge height, and relining frequency were significantly correlated with PAI. Conclusion: Implant-supported partial overdentures appear to be associated with reduced posterior mandibular alveolar ridge resorption when compared to implant-retained ones.