Purpose: To evaluate whether 6 mm long by 4 mm wide dental implants could be an alternative to implants at least 10 mm long placed in bone augmented with bone substitutes in posterior atrophic jaws. Materials and methods: A total of 20 patients with bilateral atrophic mandibles and 20 patients with bilateral atrophic maxillae, having 5 to 7 mm of bone height above the mandibular canal or below the maxillary sinus, had each side of the jaws randomly allocated according to a split-mouth design. They were allocated to receive one to three 6 mm long and 4 mm wide implants, or implants at least 10 mm long in augmented bone by two different surgeons in different centres. Mandibles were vertically augmented with interpositional equine bone blocks and resorbable barriers, and implants were placed 3 months later. Maxillary sinuses were augmented with particulated porcine bone via a lateral window and implants were placed simultaneously. All implants were submerged and loaded, after 4 months, with provisional prostheses. Four months later, definitive metal-ceramic prostheses were delivered. Outcome measures were prosthesis and implant failures, any complication and radiographic peri-implant marginal bone level changes. Results: One patient treated in the mandible dropped out before the 1-year post-loading follow-up. All maxillary implants and prostheses were successful, whereas 2 mandibular prostheses could not be placed on implants at least 10 mm long due to graft failures; one was associated with the loss of 3 implants because of infection. There were no statistically significant differences in implant and prosthesis failures, though significantly more complications occurred at grafted sites in mandibles (P = 0.0078), but not in maxillae (P = 0.1250). In total, 14 complications occurred in 12 patients at augmented sites versus none at 6 mm-long implants. All failures and complications occurred before loading. Patients with mandibular 6 mm-long implants lost an average of 1.05 mm of peri-implant bone at 1 year and patients with mandibular implants at least 10 mm long lost 1.07 mm. These differences were statistically significant (P < 0.001). Patients with maxillary 6 mm-long implants lost an average of 1.02 mm of peri-implant bone at 1 year and patients with maxillary implants at least 10 mm long lost 1.09 mm. These differences were statistically significant (P < 0.001). There were no statistically significant differences in bone level changes up to 1 year between 6 mm and at least 10 mm-long implants in both jaws (mandibles n = 18, mean difference -0.02 mm, 95% CI -0.16 to 0.12, P = 0.7384; maxillae n = 20, mean difference -0.07 mm, 95% CI -0.18 to 0.05, P = 0.2547). Conclusions: Short-term data (1 year after loading) indicate that 6 mm-long implants with a conventional diameter of 4 mm achieved similar if not better results than longer implants placed in augmented bone. Short implants might be a preferable choice to bone augmentation, especially in posterior mandibles since the treatment is faster, cheaper and associated with less morbidity. However, data obtained 5 to 10 years after loading are necessary before making reliable recommendations.