Purpose: To compare the outcome of cross-arch prostheses supported either by supershort (5 mm) or long (11.5 mm) implants, placed flapless and immediately restored with metal-resin screw-retained cross-arch prostheses. Materials and methods: Thirty patients with edentulous (or to be rendered edentulous) mandibles and 30 with edentulous maxillas, who had sufficient bone to allow the placement of four and six implants respectively, which were at least 11.5 mm-long, were randomised according to a parallel group design into 2 equal groups, where they received either 5 mm or 11.5 mm-long implants at one centre. Implants with a diameter of 5 mm, were to be placed flapless with an insertion torque of at least 50 Ncm. Mandibles received four implants between the mental foramina. Implants were to be immediately loaded with metal-resin-definitive prostheses on the same day of implant placement. Patients were followed up to 1 year after loading and the outcome measures were: prosthesis and implant failures, complications, and peri-implant marginal bone level changes. Results: No patients dropped-out. Two prostheses were remade, one on short maxillary implants and one on long mandibular implants. Two 5 mm maxillary implants which did not achieve 50 Ncm torque in soft bone of one patient, but were immediately loaded anyway, failed after 3 weeks compared to one mandibular 11.5 mm-long implant that failed after 60 days. Two complications occurred in each group. There were no statistically significant differences for prosthesis failures, implant failures and complications. Patients with mandibular short implants lost on average 0.08 mm of peri-implant bone at 1 year and patients with long mandibular implants lost 0.51 mm. Patients with short maxillary implants lost on average 0.15 mm of peri-implant bone at 1 year and patients with long maxillary implants lost 0.62 mm. Short implants showed less bone loss when compared to long implants and the differences up to 1 year were statistically significant both in maxillae (mean difference = 0.48 mm, 95% CI 0.22 to 0.73, P = 0.0011) and in mandibles (mean difference = 0.44 mm, 95% CI 0.21 to 0.66, P = 0.0009). Conclusions: Flapless-placed 5 mm-long implants achieved similar results as 11.5 mm-long implants when supporting immediately loaded cross-arch prostheses both in maxillae and mandibles up to 1 year after loading. These preliminary results must be confirmed by other trials, and 5- to 10-year post-loading data is necessary before making reliable recommendations. Keywords: edentulous jaw, flapless implant placement, immediate loading, short dental implants