Purpose: The purpose of this study was to evaluate the sealing ability of single-unit all-porcelain crowns with margins placed in cementum-dentin and luted with two different cementing materials. Materials and Methods: Twenty extracted molars, free of caries and restorations, were selected. The sample abutments were prepared in a standardized manner and then divided in two groups (n-10) at random. Group 1: The cervical margin was located 1 mm below the CEJ and the crowns were luted with EBS Multi (Espe) bonding system in combination with Compolute 1.0 Aplicap (CLA 1.0. Espe) experimental resin cement. Group 2: The cervical margin was placed 1 mm below the CEJ, and Syntac bonding system (Vivadent) and Variolink II resin cement (Vivadent) were used. IPS-Empress (Ivoclar) crowns were made following manufacturer’s instructions. After luting procedures and 2,500 thermal cycles (5° and 55° C), the samples were processed and evaluated for marginal leakage at the cervical site. After scoring dye penetration in group 1 and 2 samples, half of the sections were randomly selected in order to evaluate 1) resin cement thickness and 2) hybrid layer formation at the interface between adhesive material and dental substrate. The leakage data were statistically evaluated with the Kruskal-Wallis non-parametric analysis of variance (ANOVA) by ranks with Bonferroni alpha adjustment (p < 0.05). Results: Sixty percent of the group 1 samples and 80% of group 2 showed leakage. In group 1, the dye penetration was often limited to the cervical margin site, while in group 2, it mainly extended to the axial walls. Statistically significant differences between samples of the two groups were not found. Samples of group 1 showed a cement thickness of 32 µm, while 75 µn was found in group 2. A hybrid layer and resin tag formation were evident in group 1. In group 2 samples, the hybrid layer formation was less evident and thinner than in group 1. Conclusions: The two adhesive systems performed similarly. The cervical margins of single-unit all-porcelain crowns must be considered as one of the weakest areas of this type of esthetic restoration.