The aim of this retrospective study was todetermine the number of premature membraneexposures and to evaluate possible predisposingfactors.72 patients were treated with ePTFE membranes(GoreTex augmentation material) for membraneguided bone regeneration (GBR) of periimplantbone defects (median age 38,5 years, minimum18 years, maximum 68 years; 39 % male and 61 %female patients, data collection between 03/1993and 02/2000).61 % of the membranes were applied in the upperjaw and 39 % in the lower jaw. 61 % of all GBRprocedures were done in context with single toothreplacements.44 % of all sites revealed a premature membraneexposure (PME). 72 % of these PMEs occured afterprimary uneventful healing period after membraneimplantation.Statistical analysis revealed a significantcorrelation between the parameters 'prematuremembrane exposure', 'membrane covering thealveolar ridge' (p=0,0004)and 'membrane locatedbeneath the incision line' (p=0,0004).Additionally a tendency for correlation wascalculated for the parameters 'defectconfiguration' (p=0,08) and 'smoking' (p=0,04).The defect morphology, the flap and incisiondesign and the smoking habits of the patientshave to be taken into critical consideration ifGBR is planned. Non space maintaining defectconfigurations, smoking habits and crestalincisions crossing the planned membrane locationseem to be contraindications for the use ofePTFE membranes. However, alternative treatmentmethods should be taken into account. Schlagwörter: guided bone regeneration, complications, premature membrane exposure