Background: To evaluate dimensional bone alterations following horizontal ridge augmentation using guided bone regeneration (GBR) with or without autogenous block graft (ABG) for the rehabilitation of atrophic jaws with dental implants.Materials and Methods: Forty-two patients, with 42 severe horizontal bone atrophy sites in the maxilla or mandible were randomly assigned to two groups: ABG or GBR. The ABG group received a combination of ABG with particulate xenograft, covered by a collagen membrane, while the GBR group received particulate xenograft alone, covered by a collagen membrane.After 6-9 months of healing, implants were inserted. All implants were definitively restored 6 months after implant placement. Radiographic examination (cone-beam computed tomograms) was performed immediately after bone grafting procedure (T0), at 6 months (T6), and at 18 months (T18), to evaluate the amount of horizontal bone width (HBW) gain. Patient demographic information, amount of ridge width augmentation, implant survival, complications, and contributing factors were gathered and analyzed.Results: Thirty-nine patients completed the study. Both groups developed enough bone ridge width for implant placement. A total of 65 implants were placed. Implant survival rate was 100% in both groups at T18. Mean increases in HBW amounted to 5.6 ± 1.35 mm in GBR sites and 4.8 ± 0.79 mm in ABG sites at T18. There was no statistically significant difference in HBW gain obtained in the GBR group when compared to the ABG group at 6 months (P = 0.26) or 18 months (P = 0.26). However, the ABG group had a statistically significant higher prevalence of sensory disturbances (P = 0.02) and hematomas (P = 0.002) compared to the GBR group.