Purpose: To evaluate whether 6.6-mm long implants could be a suitable alternative to longer implants placed in vertically augmented atrophic posterior mandibles. Materials and methods: Sixty partially edentulous patients having 7 to 8 mm of residual crestal height and at least 5.5-mm thickness measured on computed tomography scans above the mandibular canal were randomly allocated according to a parallel-group design either to receive one to three submerged 6.6-mm long implants or 9.6-mm or longer implants (30 patients per group) placed in vertically augmented bone. Bone was augmented with interpositional anorganic bovine bone blocks fixed with titanium plates and covered with resorbable barriers. Grafts were left to heal for 5 months before implant placement. Four months after implant placement, provisional acrylic prostheses were delivered, replaced, after 4 months, by definitive metal-ceramic prostheses. Outcome measures were: prosthesis and implant failures, complications, and radiographic peri-implant marginal bone level changes. Patients were followed up to 8 years after loading. Results: Eight years after loading 12 patients dropped out, five from the short implant group and seven from the augmented group. The augmentation procedure failed in two patients and only 6.6-mm long implants could be inserted. There were no statistically significant differences for prosthesis and implant failures. Four prostheses failed in three patients of the short implant group versus three prostheses in three patients of the augmented group (Fisher exact test P = 1.000; difference in proportions = 0.01; 95% CI: −0.19 to 0.22). Five short implants failed in three patients versus three long implants in three patients (Fisher exact test P = 1.000; difference in proportions = 0.01; 95% CI: −0.19 to 0.22). There were statistically more complications in augmented patients (27 complications in 22 augmented patients versus 9 complications in 8 patients of the short implant group) (Fisher exact test P < 0.001; difference in proportions = 0.64; 95% CI: 0.38 to 0.79). Both groups gradually lost peri-implant bone in a statistically significant way. Eight years after loading, short implant group patients lost an average of 1.58 mm of peri-implant bone compared with 2.46 mm in the augmented group. Short implants experienced statistically significantly less bone loss (0.88 mm, 95% CI: 0.50 to 1.26 mm) than long implants. Conclusions: When residual bone height over the mandibular canal is between 7 and 8 mm, 6.6-mm short implants are an interesting alternative to vertical augmentation in posterior atrophic mandibles since the treatment is faster, cheaper and associated with less morbidity.