Purpose: To evaluate whether 4.0 x 4.0 mm dental implants could be an alternative to implants at least 8.5 mm long, which were placed in posterior jaws, in the presence of adequate bone volumes. Materials and methods: One hundred and fifty patients with posterior (premolar and molar areas) jaws having at least 12.5 mm bone height above the mandibular canal or 11.5 mm below the maxillary sinus, were randomised according to a parallel group design, in order to receive one to three 4.0 mm-long implants or one to three implants which were at least 8.5 mm-long, at three centres. All implants had a diameter of 4.0 mm. Implants were loaded after 4 months with definitive screwretained prostheses. Patients were followed up to 1-year post-loading and outcome measures were prosthesis and implant failures, any complications and peri-implant marginal bone level changes. Results: Seventy-five patients were randomly allocated to each group. One patient dropped out after the 4-month post-loading evaluation from the long implant group. Up to 1-year post-loading, 3 patients lost one 4.0 mm-long implant each in comparison to 2 patients who lost one long implant each (difference in proportion = 0.013; 95% CI: -0.058 to 0.087; P = 0.506). All failures occurred before loading, the failed implants were replaced and the delivery of two prostheses in each group was delayed for several months (difference in proportion = 0.0004; 95% CI: -0.068 to 0.069; P = 0.685). Three short implant patients experienced three complications versus 2 long implant patients (difference in proportion = 0.013; 95% CI: -0.058 to 0.087; P = 0.506). There were no statistically significant differences in prosthesis failures, implant failures and complications. Patients with short implants lost on average 0.53 mm of peri-implant bone and patients with longer implants lost 0.57 mm. There were no statistically significant differences in bone level changes up to 1 year between short and long implants (mean difference = 0.038 mm; 95% CI: -0.068 to 0.138; P = 0.198). Conclusions: One year after loading 4.0 mm-long implants achieved similar results as 8.5 mm-long or longer implants in posterior jaws, however 5- to 10-year post-loading data are necessary before reliable recommendations can be made. Keywords: short dental implants, posterior jaws