Purpose: The aim of this study was to evaluate the effect of treatment with either two or four mandibular endosseous implants with an overdenture on mandibular posterior residual ridge resorption over a 10-year period. Materials and Methods: Sixty edentulous patients with residual mandibular height between 12 and 18 mm participated. Thirty patients were treated with an overdenture supported by two IMZ implants (group A) and 30 patients were treated with an overdenture with four IMZ implants (group B). Before treatment and 10 years after treatment, panoramic radiographs were taken and compared to ascertain possible bone loss. Proportional area measurements were used to determine changes in the mandibular posterior residual ridge bilaterally. Results: There was a statistically significant difference in mandibular posterior residual ridge resorption between the two treatment protocols. The posterior bone area index was reduced by a mean of 10% for group A and 6% for group B over 10 years. Conclusions: There was a significant difference in mandibular posterior residual ridge resorption between patients treated with either two or four implants to stabilize an overdenture. No correlation was shown between mandibular posterior residual ridge resorption and peri-implant marginal bone loss. The confounding factors of marginal bone loss around the implants, age, gender, initial mandibular height, and the number of years the patient had been edentulous failed to show a significant effect on posterior ridge resorption. Keywords: bone resorption, endosseous dental implants, mandible, overdenture, residual ridge