Purpose: To evaluate whether 6 mm-long by 4 mm-wide dental implants could be an alternative to at least 10 mm-long implants placed in bone augmented with bone substitutes in posterior atrophic jaws. Materials and methods: Twenty 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, were randomised according to a split-mouth design to receive one to three 6 mm-long and 4 mm-wide implants or at least 10-mm long implants in augmented bone at two centres. Mandibles were vertically augmented with interpositional equine bone blocks and resorbable barriers, and implants were placed after 3 months. 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 screw-retained metal-ceramic prostheses were delivered. Outcome measures were prosthesis and implant failures, any complication, time needed to fully recover mental nerve function (only for mandibular implants) and patient preference. Results: All screened patients had sufficient bone width to support 4 mm-diameter implants. Patients were followed up to 5 months post-loading and none dropped out. There were no statistically significant differences in graft, implant or prosthesis failures, though significantly more intra- and postoperative complications occurred at grafted sites. Fourteen complications occurred in 12 patients at augmented sites versus none at short implants. All complications occurred before loading. Three complications were associated with the failure of the mandibular grafts (15%), determining the failures of 3 implants in one patient and 2 prostheses could not be delivered. One patient was re-grafted and 2 patients received short implants instead. Apart from those complications associated with graft failures, there were 4 perforations of the sinus membrane during sinus lifting and 7 temporary lower lip paraesthesiae lasting up to 4 days with no long-term consequences for the patients. All maxillary implants and prostheses were successful. All 20 patients treated with mandibular implants and 15 patients treated with maxillary implants preferred short implants, whereas 5 patients treated with maxillary implants described both procedures as equally acceptable. These differences were statistically significant. Conclusions: Short-term data (5 months 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, 5- to 10-year post-loading data are necessary before making reliable recommendations. Schlagwörter: bone substitutes, inlay graft, short dental implants, sinus lift, vertical augmentation