Hegazy S. A.¹, Khairallah A. S.² and Youssef S. A^{.3 1} Professor of Removable Prosthodontic, Faculty of Dentistry, Mansoura University, Egypt. ²Assistant Prof.ofRemovable Prosthodontic, Faculty of Dentistry, Mansoura University, Egypt. ³ Graduate student, Department of Prosthodontic, Faculty of Dentistry, Mansoura University, Egypt. Abstract: Statement of problem:The effect of different ball diameters of ball and socket retained mandibular overdenture on mandibular bone resorption has insufficient data. Purpose: This study aimed to evaluate the marginal bone hieght changes and the posterior ridge area index of a mandibular overdenture retained by two different sizes of ball and socket attachments. Materials and methods:Six male patients with reduced stability and insufficient retention of mandibular dentures received 2 implants in the interforminal region. Implants were connected to mandibular overdentures with ball and socket attachments. According to the ball diameter, the patients were classified into two groups: Group I received 2.5mm ball diameter and group II received 1.8mm ball diameter. Marginal bone height changes and posterior ridge area index were evaluated 6 months after overdenture insertion (T1), 12 months (T2) and 18 months (T3).  Results: Marginal bone resorption was measured through the evaluation period in both groups. Group I showed (0.79),(1.3) and (1.7) values regarding the vertical  marginal   bone loss respectively and showed (0.45), (0.68)  and (0.98) values regarding the horizontal marginal bone loss respectively. Group II showed (0.41),(0.67) and (1.1) values regarding the vertical  marginal   bone loss  respectively and showed (0.17),(0.28) and (0.39) values regarding the horizontal marginal bone loss respectively. Regarding the posterior area index, group I recorded (1.6), (1.7) and (1.8) values respectively. Group II recorded (1.5), (1.6) and (1.7) values respectively through the evaluation period (6,12 and 18 months). There was statistically insignificant difference between groups regarding the posterior area index at all observation times. Conclusion: Ball and socket retained mandibular overdentures with a 1.8mm ball diameter were associated with less vertical and horizontal bone resorption compared with 2.5 mm ball diameter after a follow up of 1.5 years.