AIM: To investigate the impact of different vertical interimplant distances (VIDs) on the long-term success of two adjacent single-unit implants. MATERIALS AND METHODS: This retrospective study categorized implants into two groups based on VID: Group 1 (VID <
2 mm) and Group 2 (VID ≥ 2 mm). Further classification was done according to horizontal inter-implant distance (HID). Statistical models were used to evaluate the primary outcome of implant survival, secondary outcomes of complication-free survival rates and radiological findings. RESULTS: We included 290 patients with 376 two adjacent single-unit implants. Group 1 included 223 patients with 280 implants, whereas Group 2 comprised 89 patients with 96 implants. The implant survival rates were 93.2% in Group 1 and 96.8% in Group 2 over the 8 years, with no significant association between the groups. Similarly, the overall complication-free survival rates were 60.6% in Group 1 and 65.6% in Group 2, showing no significant association between the groups. Radiological assessments also showed no notable association between the groups. This trend was consistent in the subgroup analyses based on HID, with no significant differences in implant survival, complication-free survival rates or radiological outcomes. CONCLUSIONS: Considering the study limitations of an 8-year post-prosthetic review period and despite varying VIDs, our findings showed no significant difference in implant survival, complication-free survival rates or radiological assessments. This underscores the positive prognosis of two adjacent single-unit implants and contributes to a deeper understanding of the success factors in implantology. TRIAL REGISTRATION: The study was registered in the Chinese Clinical Trial Registry (ChiCTR2300073215).