Mohammed A.Elsawy*, Salah A.Hegazy** ,AshrafGebreel*** ^{*BDS, faculty of dentistry ,Mansoura university ** Prof .of prosthodontics, faculty of dentistry ,Mansoura university *** Lecture of prosthodontics, faculty of dentistry ,Mansoura university}   Purpose: The aim of the present study was evaluation of retention and peri-implant outcomes on quadrilateral four Locator attachment retained mandibular overdenture. Material and methods: six patients were selected for this study with average age from 45-55.They were completely edentulous patients. Complete dentures were constructed and CAD/CAM limiting surgical stereo lithographic surgical guide was constructed. According to distribution of implants, the patients received four interforaminal implants located at the lateral incisor and first premolar region in quadrilateral design. After osseointegration period, locator attachments were used to retain the mandibular overdenture. The retention of the mandibular overdenture and peri-implant outcome were evaluated using the following variables (1) modified plaque index ;(2) modified bleeding index;(3)probing depth. The measurements were evaluated after one month of mandibular overdenture insertion (T0), threemonths (T2), sixmonths (T3) after mandibular overdenture insertion. Results: There was statistically insignificant difference of retention measurements after three months of denture insertion while there was significant decrease of retention after six months of denture insertion.(Statistically significant p < 0.05).The peri-implant outcome showed there was statistically significance increase of modified plaque index and probing depthwith the advancement of time. And significance decrese of bleeding index with advancement of time. Conclusion: 1-Placing the implants in quadrilateral distribution in the inter-foraminal area at the lateral incisor and first premolar region is a reliable treatment when maximum retention is needed. 2_The quadrilateral design showed preservation of the implants regarding the peri-implant outcome over a period of study.