Purpose: To evaluate the outcome of one (fixed-on-1, Fo1) versus two (fixed-on-2, Fo2) implants placed flapless in fully edentulous mandibles and immediately restored with metal-resin screw-retained cross-arch prostheses. Materials and methods: Twenty patients with edentulous or to be rendered edentulous mandibles were randomised to the Fo1 group (10 patients who received three intraforaminal implants but only the mesial implant was actually loaded) and to the Fo2 group (10 patients who received two intraforaminal implants) according to a parallel-group design. To be immediately loaded implants had to be inserted with a minimum torque of 60 Ncm. Outcome measures were prosthesis and implant failures, complications and patient satisfaction for function and aesthetics evaluated up to 10 months post-loading. Results: Flaps were raised in three patients of the Fo1 group and one of the Fo2 group. Two prostheses in each group were delayed loaded at 2 and 3 months, because implants could not be inserted with a torque superior to 60 Ncm. Patients of the Fo2 group received prostheses with 12 teeth whereas Fo1 patients with 8 or 10 teeth. Ten months after loading no drop-out or implant failure occurred, but one Fo2 prosthesis had to be remade because the patient complained it was too short (risk difference = 0.1; 95% CI: -0.19 to 0.40; Fisher's exact test P = 1.000). Three patients were affected by complications in each group (risk difference = 0.0; 95% CI: -0.359 to 0.359; Fisher's exact test; P = 1.000). There were no statistically significant differences for prosthetic failures and complications between groups. Four months after loading, patients of the Fo1 group were less satisfied for function than those of the Fo2 group (difference between the medians [Hodges-Lehmann estimation] = 1; 95% CI: 0 to 2; P = 0.043). No differences were observed for aesthetics (difference between the medians [Hodges-Lehmann estimation] = 0; 95% CI: -1 to 0; P = 0.481), and two Fo1 patients would not undergo the same procedure again (difference in proportions: 0.2; 95% CI: -0.11 to 0.51; P = 0.474). Conclusions: Preliminary results suggest that immediately loaded cross-arch screw-retained fixed prostheses with 8 to 10 teeth can be supported by only one dental implant at least up to 10 months post-loading; however, the clinical application and usefulness of such a procedure remains questionable.