Clinical experience has shown that CAD/CAM produced ceramic restorations using “Dental Database” mode usually require intra-oral adjustment of the occlusal surfaces.  Objective: This study measured the vertical discrepancy, in a positive or negative direction, of a CAD/CAM generated occlusal surface on a MODB onlay derived with the “Correlation” mode and “Dental Database” mode.  “Correlation” mode involves developing the occlusal morphology from an intact tooth or constructed from a waxed pattern of the occlusal surface of the tooth being restored.  The “Dental Database” mode utilizes the morphological knowledge contained in the CEREC 3 dental database.  Methods: Twenty MODB CAD/CAM (CEREC 3) ceramic onlays were fabricated from a prepared typodont mandibular first molar.  Ten of the restorations were manufactured in the “Correlation” mode recorded from the unprepared typodont tooth.  Ten restorations were made in the “Dental Database” mode.  The fabricated ceramic onlays were placed into the cavity preparation without any luting cements.   No adjustment was necessary in the seating of the milled onlays.  The hyper-occlusion (positive value) was evaluated by placing the autopolymerizing resin (Pattern Resin, GC America) between the incisal pin and the flat anterior guide table, and measured the thickness of the resin pattern with a digital micrometer (Mitutoyo Absolute Digimatic, Mitutoyo Corp., Japan).  Results: The mean vertical discrepancy for “Correlation” mode (0.24 ± 0.021 mm) was statistically different (Student t-test, p £ 0.001) from “Dental Database” mode (1.00 ± 0.051 mm). No samples demonstrated infra-occlusion.   Conclusion: Within the experimental design tested, the occlusal surface of onlays generated with the “Correlation” mode showed significantly less vertical discrepancy than the  “Dental Database” mode.