Purpose: This study investigated the effects of certain systemic and local factors on resorption of the posterior mandibular residual ridge under conventional dentures and overdentures supported by 2 implants. Materials and Methods: Proportional area measurements of the posterior mandible were made on rotational tomograms taken immediately before and 5 years after treatment. The area was bounded by a line joining gonion to the lowest point of the mental foramen and the crest of the residual ridge and was expressed as a proportion of an area that was not dependent on the ridge. The use of proportions rather than actual measurements minimized errors related to magnification and distortion. Results: The estimated average reduction in height was 1.25 mm in 5 years (1.63 mm for conventional denture groups and 0.69 mm for implant overdenture groups, ie, almost 1 mm less in the overdenture group). Discussion and Conclusion: Female gender was a risk factor for greater resorption. Other factors, such as the number of years a patient had been edentulous, initial height of the mandible, and the number of dentures used, failed to show an association with resorption of the residual posterior mandibular ridge, while the statistically significant effect of age was unlikely to be clinically significant.