OBJECTIVE To compare by confocal and scanning electron microscopy the post-resin-dentin interfaces of endodontically-treated teeth which had been restored with dentin-bonding systems and composite and needed to be extracted after 6 months to 6 years of clinical service. METHODS Sixteen single rooted teeth showing proximal lesions penetrating the pulp chamber were included in the investigation and divided into two groups. Group 1 teeth were endodontically-treated, restored with carbon fiber posts and resin-based composite. Group 2 teeth were treated in the same way and covered with porcelain fused-to-metal crowns. RESULTS Two thirds of the observed interfaces showed resin penetration of the root dentin in the middle and coronal third of the post space preparation, while only one third of the interface showed resin infiltration in the apical third of the post space preparation. Debonding of the adhesive from the resin infiltrated dentin area and debonding of the composite cement from the adhesive were the most frequently observed failure modes and were observed in one third of the observed interfaces. There was no difference between the post-resin-dentin interfaces of teeth covered and teeth not covered with porcelain-fused to metal crowns. Teeth extracted after longer periods of clinical service showed more abundant areas of debonding of the adhesive from the resin-dentin interdiffusion zone. The presence of bacteria in resin infiltrated dentin tubules was observed in two specimens.