Purpose: To compare the clinical outcome of fixed prostheses supported by 4- to 8-mm-long implants with prostheses supported by longer implants placed in vertically augmented atrophic mandibles after a follow-up of 5 years in function. Materials and methods: The Cochrane Central Register of Controlled Trials (CENTRAL) and MEDLINE were searched up to 1st September 2018 for randomised controlled trials (RCTs) with a follow-up of at least 5 years in function comparing fixed prostheses supported by 4- to 8-mm-long implants with prostheses supported by longer implants placed in vertically augmented atrophic mandibles. Outcome measures were prosthesis failure, implant failures, augmentation procedure failures, complications, and peri-implant marginal bone level changes. Screening of eligible studies, assessment of the risk of bias and data extraction were conducted in duplicate and independently by two review authors. The statistical unit of the analysis was the prosthesis. Results were expressed as random-effects models using mean differences for continuous outcomes and risk ratios (RR) for dichotomous outcomes with 95% confidence intervals (CIs). Results: Four eligible RCTs that included originally 135 patients were included. Two RCTs had a parallel-group design and two a split-mouth design. Short implants were 5.0 to 6.6 mm long and were compared with longer implants placed in posterior mandibles augmented with interpositional blocks of bone substitutes. All trials were judged at unclear risk of bias. Twelve (14%) bone augmentation procedures failed to achieve the planned bone height to allow placement of implants with the planned length. Five years after loading, 28 patients (21%) had dropped out from the four RCTs. There were no differences for patients having prosthesis (RR = 1.46; 95% CI: 0.52 to 4.09; P = 0.47; I2 = 0%) or implant (RR = 1.00; 95% CI: 0.31 to 3.21; P = 1.00; I2 = 0%) failures between the two interventions, but there were more patients experiencing complications (RR = 4.72; 95% CI: 2.43 to 9.17; P < 0.00001; I2 = 0%) and peri-implant marginal bone loss (mean difference = 0.60 mm; 95% CI: 0.36 to 0.83; P < 0.00001; I2 = 45%) at longer implants in augmented bone. Conclusions: Five years after loading, prosthetic and implant failures were similar between the two interventions, but complications and peri-implant marginal bone loss were higher and more severe at longer implants placed in vertically augmented mandibles. Larger trials and longer follow-ups up to 10 years after loading are needed to confirm or reject the present preliminary findings. However in the meantime short implants could be the preferable option.