The composite luting gap between ceramic and dental hard tissue can be termed an "Achilles heel". Therefore, one major goal of luting ceramics focuses on minimizing the inter-marginal gap area. This study evaluated the marginal accuracy of two all-ceramic systems. The null hypothesis was that there is no statistical difference between the marginal accuracy of the IPS Empress and Cerec 3D all-ceramic systems. On 16 casts, representing different clinical situations, the left first mandibular molar was prepared to receive large onlays (MOD and replacement of the distobuccal and distal cusps). For each cavity, one laboratory heat-pressed (IPS Empress) and one chairside CAD/CAM restoration (Cerec 3D) were manufactured. A newly developed milling unit was used for CAM processing. The restorations were placed in their respective cavities and die replicas were taken and examined under SEM for quantitative gap measurement. The gap width was measured at 11 defined landmarks by two different examiners. An overall gap width of 56 microm (+/- 31 microm) was measured for IPS Empress, compared to the significantly increased value of 70 pm (+/- 32 microm) for Cerec 3D. From a clinical viewpoint, the statistically significant difference between the two systems is not relevant, since both systems still exhibit a clinically acceptable gap width of less than 100 microm.