This study aimed to evaluate fractures that attacked mandibular overdentures retained by two implants with bar clip system and correlates them with the existing mandibular space, thickness of the prosthesis and masticatory efficiency. This is a retrospective cohort study with a sample of 23 patients toothless bimaxillary users double conventional dentures which had their lower dentures converted to removable dentures on implants - overdentures. All patients were rehabilitated in Clinical Prosthodontics and Maxillofacial Surgery Department of Dentistry (DOD) in the Federal University of Rio Grande do Norte (UFRN) from January 2011 to April 2014. The mandibular space was measured with the aid of a millimeter ruler and needle point. The thickness of the base of the overdenture was checked with use of a manual espessímetro at a specific point. Masticatory efficiencies were collected from a database of a previous search that used the colorimetric method with fuscina capsule. Survival analysis of Kaplan-Meier and the T test to analyze the independent variables was used. There were eight fractures which represents a total of 34.7% of the sample. The found survival rate was 49.5% and were not found significant values (p <0.05) variables related to occurrence of fracture (mandibular space, thickness of the denture base and masticatory efficiency). It was concluded that it is technically difficult to prevent such an occurrence, so it is suggested strengthening the prosthesis structure as part of the protocol in its manufacturing process.This study aimed to evaluate fractures that attacked mandibular overdentures retained by two implants with bar clip system and correlates them with the existing mandibular space, thickness of the prosthesis and masticatory efficiency. This is a retrospective cohort study with a sample of 23 patients toothless bimaxillary users double conventional dentures which had their lower dentures converted to removable dentures on implants - overdentures. All patients were rehabilitated in Clinical Prosthodontics and Maxillofacial Surgery Department of Dentistry (DOD) in the Federal University of Rio Grande do Norte (UFRN) from January 2011 to April 2014. The mandibular space was measured with the aid of a millimeter ruler and needle point. The thickness of the base of the overdenture was checked with use of a manual espessímetro at a specific point. Masticatory efficiencies were collected from a database of a previous search that used the colorimetric method with fuscina capsule. Survival analysis of Kaplan-Meier and the T test to analyze the independent variables was used. There were eight fractures which represents a total of 34.7% of the sample. The found survival rate was 49.5% and were not found significant values (p <0.05) variables related to occurrence of fracture (mandibular space, thickness of the denture base and masticatory efficiency). It was concluded that it is technically difficult to prevent such an occurrence, so it is suggested strengthening the prosthesis structure as part of the protocol in its manufacturing process.