Objectives: The aim of this study was to evaluate the effect of ceramic in the Knoop hardness of the resin cement, after polymerization with three light curing units. Methods: Sixty bovine incisors were flattened with 600 grit SiC paper to expose a flat dentin area of 5mm in diameter. The dentin was treated with Prime&Bond NT (Dentsply) adhesive system. After, the resin cement Enforce (Dentsply) was inserted into a rubber mould and light-cured through a feldspatic ceramic HeraCeram (Kulzer) disk of 8mm in diameter and 1.5mm in thickness, simulating the presence of a indirect restoration, using the light curing units: quartz Tungsten Halogen Light-QTH (XL 2500, 3M/ESPE) for 40s; light emitted by diode-LED (Ultrablue Is, DMC) for 40s, and Xenon Plasma Arc - PAC (Apolo 95E, DMD) for 3s. The resin cement specimens were flattened to their middle portion and Knoop hardness was measured using HMV 2 microhardness tester (Shimadzu), with 50g load for 15s, after storage at 37°C for 24 hours. Fifteen measurements were taken for each specimen, with 20 specimens for each lightcuring unit (10 for control group-direct lightcuring and 10 for indirect lightcuring). Data were submitted to ANOVA and Tukey's test (p=0.05). Results: The Knoop hardness of the control groups and of indirect lightcuring were, respectively: QTH (45.0; 41.7 KHN); LED (53.6; 50.1 KHN); PAC (39.1; 38.1 KHN). LED showed statistically higher Knoop hardness than QTH and PAC for control group and indirect lightcuring. Significant statistical differences were found between specimens of the control group and of indirect lightcuring with QTH and LED. No statistical difference was found for PAC. Conclusions: The Knoop hardness of the resin cement decreased significantly with the interposition of the ceramic and was influenced by lightcuring units.

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