Purpose: The purpose of this study was the clinical and radiographic evaluation of four-implant-supported fixed prostheses and milled bar overdentures for rehabilitation of the edentulous mandible. Materials and Methods: Thirty-six edentulous participants received four implants in the mandible (two vertical implants in the canine/lateral incisor area and two distally inclined implants anterior to the mental foramina) using flapless surgery. The implants were loaded with the mandibular dentures on the same day after necessary modifications. Three months after implant placement, participants were randomly allocated to one of two groups: (1) the overdenture group, where participants received milled bar overdentures; or (2) the fixed group, where participants received ceramo-metal fixed prostheses. Plaque and gingival indices, pocket depth, implant stability (using Osstell device), and marginal bone resorption (using standardized intraoral radiographs) were evaluated at the time of prosthesis insertion (T0), and 6 (T6) and 12 (T12) months after insertion. Results: The implant survival rate was 100% for both groups. Plaque Index, Gingival Index, pocket depth, implant stability, and bone resorption significantly increased by time for anterior (P .001) and posterior (P .018) implants. Fixed prostheses showed significantly higher Plaque Index, Gingival Index, and pocket depth than milled bar for anterior (P .001) and posterior (P .037) implants. No significant differences in implant stability and bone resorption between groups were noted. For fixed prostheses, anterior implants showed significantly higher Plaque Index, Gingival Index, and pocket depth than posterior implants (P .001) after 12 months. However, no significant differences in implant stability and bone resorption between anterior and posterior implants were noted for both groups. Conclusion: Both fixed prostheses and milled bar prostheses could be used successfully for immediately loaded four-implant rehabilitations of the edentulous mandible, as they were associated with favorable clinical and radiographic outcomes after 1 year. However, milled bar may be more advantageous than fixed prostheses in terms of reduced plaque/ gingival indices and probing depth. Keywords: All-on-4, bone resorption, clinical, implant, milled bar, prosthesis