Statement of Problem: Several dilemmas have been reported with regard to the retention and longevity of implant-retained overdentures. A few studies have investigated the influence of implant and attachment inclination on the path of insertion and withdrawal of the prosthesis and on the retention and longevity of the overdenture. However, no study has been reported with regard to the influence of labio-lingual inclination on the aforementioned indices.Purpose: The purpose of this study was to investigate the influence of implants and attachments with 5 and 10 degrees of inclination on the retention and longevity of implant-retained overdentures.Materials and Method: In this experimental study, 10 implants and 50 attachments were selected and divided into five groups: Group I: two implants and attachments parallel to each other; Group II: implants inclined at an angle of 5 degrees and attachments without any inclination; Group III: implants and attachments inclined at an angle of 5 degrees; Group IV: implants inclined at an angle of 10 degrees and attachments without any inclination; Group V: implants and attachments inclined at an angle of 10 degrees. All the attachments and implants were lubricated by artificial saliva. Initial retention (N) of blocks was measured through the Universal Testing Machine (SANTAM, STM-20). The blocks were removed and replaced for 3000 cycles and retention was measured after each 500 cycles. The retention was measured five times for each group and the registered data were analyzed by One-way ANOVA and T-test.Results: The maximum and minimum amounts of initial retention were 5.54±0.3 and 3.88±o.19 and were related to groups III and I, respectively. There was no significant difference among groups II, III, IV, and V with regard to the amount of retention of implants and attachments ( p < 0.3). However, there was a significant difference between the control group, group I, and the other groups ( p <0.01).Conclusion: Although the placement of labially inclined implants results in an increase in the initial retention, it will lead to a large decrease in the amount of retention after the last cycle. However, this amount of inclination (5 or 10 degrees) does not have any negative effect on the prosthesis longevity.