Aim. The aim of the present microleakage study was to assess the sealing ability of nanohybrid composite crowns with different finish lines exposed to simulated mechanical periodontal treatment (SMPT). Methods. After sample size calculation (\u3b1=0.05; \u3b2=0.20; \u3b4=1.0; \u3c3=0.8), sixty extracted mandibular molars were divided into four groups (N.=15): G1, 90\ub0 shoulder; G2, beveled 90\ub0 shoulder; G3, 90\ub0 shoulder and SMPT; G4, beveled 90\ub0 shoulder and SMPT. Tooth preparations were carried out by means of diamond burs and Arkansas stones. The buildup of crowns was performed with a nanohybrid composite on master casts obtained after polyether impressions and crowns were cemented with self-adhesive cement. Groups G3 and G4 were subjected to the equivalent of five years of semestral mechanical periodontal scaling with Gracey curettes (2-mm long strokes, 5 N). Samples were immersed into a methylene blue supersaturated solution for 10 minutes. Microleakage was measured by stereomicroscopic observation of multiple sections of the samples and leakage data underwent statistical analysis with non-parametric tests. Results. Marginal microleakage was 1.53\ub11.27% and 17.60\ub112.72% of the length of the adhesive interface in G1 and G2, respectively. SMPT reduced dye penetration (P<0.001) with G3 not leaking at all and G4 leaking along the 5.58\ub11.84% of the adhesive interface. The bevel preparation significantly worsened the marginal seal both in control and treated crowns (P<0.001). Conclusion. Microleakage of nano hybrid composite crowns increased by adding a bevel to a 90\ub0 shoulder preparation and diminished after SMPT
