One of the key factors for attaining osseointegration is the presence of an adequate osseous volume. In patients with inadequate osseous width or height, a bone augmentation using the guided bone regeneration (GBR) concept may be applied either with a simultaneous or a staged approach. The aim of this multicenter prospective case series study was to evaluate the efficacy and predictability of the GBR technique (simultaneous approach) in patients with peri-implant osseous defects, both dehiscences and fenestrations. Results 5 years post-treatment (survival rates and marginal bone level) were assessed. A total of 19 consecutive patients with 26 peri-implant osseous defects (20 dehiscences and six fenestrations) were treated during the period from September 1992 to June 1993 with a simultaneous GBR approach using non-resorbable membranes combined with autogenous bone grafts or decalcified freeze-dried bone allograft. The mean osseous augmentation was 94.8%. Marginal bone levels at re-entry and 5 years after surgery were calculated from standardized periapical radiographs. One implant was lost 3 months after loading. Thus, the cumulative survival rate was 96.1% after 5 years. The mean marginal bone level after 5 years was 2.03 mm (SD=+/-0.5), without a difference between mesial and distal sites. This study demonstrates that implants with peri-implant defects that are treated with GBR had similar survival rates and crestal bone levels compared with implants in native bone.