Purpose: There is a growing pressure in implant dentistry to perform implant rehabilitations in difficult conditions, such as fresh extraction sockets, periodontally compromised sites, or sites with low bone density and quantity that imply a high likelihood of dehiscences or fenestrations. The aim of this study was to document complete rehabilitations in both jaws through the so-called All-on-Four concept (ie, four implants with the posterior implants placed at an angle) using immediate function implants inserted in "nonideal" conditions (eg, implants inserted with dehiscences or fenestrations, in periodontally compromised sites, or in fresh extraction sockets). Materials and Methods: This prospective cohort study included patients with immediately functioning implants placed in difficult conditions to support fixed full-arch maxillary and mandibular prostheses. Results: One hundred forty-two patients received 227 implants. A total of seven implants failed in six patients (three implants in three patients in the maxilla and four implants in three patients in the mandible), for cumulative patient and implant survival rates of 96.2% and 97.7% for the maxilla and 94.7% and 94.8% for the mandible, respectively, after 2 years (Kaplan-Meier). The average (standard deviation) peri-implant bone resorption after 1 year and 3 years in the maxilla was 1.3 mm (0.4 mm) and 1.6 mm (0.4 mm), respectively; in the mandible, the average (standard deviation) peri-implant bone resorption at 1 year and 5 years was 1.4 mm (0.3 mm) and 1.7 mm (0.6 mm), respectively. Conclusions: The results support the conclusion that the four-implant ("All-on-Four") immediate-function concept in completely edentulous maxillae and mandibles using implants inserted in difficult conditions is a viable concept in the short term. Keywords: All-on-Four, dehiscence, fenestration, immediate function, periodontally compromised, postextraction