Purpose: To compare outcomes of immediate non-occlusal loading with delayed implant loading in the bilateral replacement of mandibular first molars. Materials and methods: This study was designed as a split-mouth, randomised controlled trial. Twenty patients with bilaterally missing mandibular first molars randomly received immediately or conventionally loaded single implants. One molar was restored with a non-occlusal temporary crown within 24 hours after implant placement (immediate loading group, IL) while the contralateral molar was restored with a definitive crown 4 to 5 months later (delayed loading group, DL). A total of 40 implants were installed. All implants were inserted in healed bone with an insertion torque between 35 and 45 Ncm. Outcome measures were implant failure, complications, radiographic marginal bone level changes, probing pocket depths (PPDs) and bleeding on probing (BOP). Clinical data were collected at implant placement, and after 6, 12 and 60 months. Results: No patients dropped out and no implant failed. Only minor prosthetic complications were observed (two provisional acrylic crown fractures in the IL group and four ceramic chipping in the DL group). Two patients had bilateral peri-implant mucosal inflammation with BOP after 6 months. The differences between groups were not statistically significant (OR = 0.500; 95% CI: 0.045 to 3.489; P = 0.6831). At the 1-year follow-up examination, the mean marginal bone level was 0.83 ± 0.16 mm (95% CI: 0.75 to 0.91) in the IL group and 0.86 ± 0.16 mm (95% CI: 0.78 to 0.94) in the DL group, with no statistically significant differences between groups (difference = 0.03 ± 0.15 mm; 95% CI: −0.07 to 0.07; P = 0.53). After 5 years, mean marginal bone level was 1.06 ± 0.38 mm (95% CI: 0.97 to 1.15) in the IL group and 1.07 ± 0.32 mm (95% CI: 0.95 to 1.16) in the DL group, with no statistically significant differences between groups (difference = 0.01 ± 0.22 mm; 95% CI: −0.10 to 0.10; P = 0.96). The mean marginal bone loss after 5 years was 0.62 ± 0.45 mm in the IL group and 0.69 ± 0.33 mm in the DL group (difference = 0.07 ± 0.32 mm; 95% CI: −0.10 to 0.18; P = 0.567). At the 5-year follow-up the mean PPD and BOP values were 2.82 ± 0.65 mm and 1.17 ± 0.92 in the IL group, and 2.85 ± 0.53 mm and 1.17 ± 0.86 in the DL group, respectively. No significant differences were found (difference = 0.03 ± 0.15 mm; 95% CI: −0.15 to 0.21; P = 0.990; and 0.01 ± 0.07; 95% CI: −0.06 to 0.08; P = 1.000, respectively). Conclusions: Within the limitations of this study, the present data seem to confirm the hypothesis that the clinical outcome of immediate versus delayed loading of implants in mandibular fist molar sites is comparable. Keywords: delayed loading, dental implants, immediate loading, single mandibular tooth The authors deny any conflict of interest. This study was partially supported by Nobel Biocare. Grant: 2007 646.