Purpose: This prospective randomized controlled trial aimed to compare single implant-supported mandibular molar restorations using either an immediate or a delayed loading protocol. Materials and Methods: Thirty subjects requiring single mandibular molar replacement were consecutively treated. One implant was placed in each patient. Fifteen subjects were assigned to delayed loading protocol and 15 to immediate loading protocol according to a randomization table. After insertion, the delayed loaded implants were connected to a healing abutment and restored after 3 to 4 months of healing without loading. The immediately loaded implants were loaded within 24 hours of surgery with a provisional restoration. The interim prosthesis was placed in centric occlusion. All contacts in lateral excursions were eliminated. At implant placement the maximum value of insertion torque was recorded. Radiographic bone level change was measured on periapical radiographs obtained at the time of implant placement and 12 months after loading. Means of the 2 groups were compared by Student t test and analysis of variance (ANOVA). The level of significance was set at .05. Results: No implants were lost in the delayed loading group (0/15), whereas 1 implant failed (1/15) in the immediate loading group. No differences were observed in relation to implant length or insertion torque between the groups. The average radiographic bone level change after 1 year of function was 1.2 ± 0.55 mm (range, 0.5 to 2.6 mm) and 0.77 ± 0.38 mm (range, 0.29 to 1.23 mm) for the delayed loaded and the immediately loaded implants, respectively. The difference in radiographic bone level change between the delayed and immediate loading groups was statistically significant (P = .022; CI = -0.79 to -0.06; Student t test). Conclusions: Immediate loading of wide-diameter implants supporting single restorations in mandibular molar sites seems to be a suitable clinical option. Moreover, the radiographic bone level change observed after 12 months of loading was significantly less for immediately loaded implants. Schlagwörter: dental implants, immediate loading, randomized controlled trial