Objective: This in vitro study examined the effect of three commercial hydrogen peroxide-based home bleaching agents on enamel microhardness. Methods: Fifteen human molars were sectioned mesio-distally. The bucal and lingual surfaces were included in self curing acrylic resin, the specimens were ground to 600 grid and randomly assigned to 5 groups (n=6) according to home-bleaching agent: 4.5% hydrogen peroxide, Perfecta (Per.Premier); 5.5% hydrogen peroxide, DayWhite (DW5.Discus Dental); 7.5% hydrogen peroxide, DayWhite (DW7.Discus Dental), polyethylene-strip whitening system containing 5.3% hydrogen peroxide, Whitestrips (Whi.Procter&Gamble) and 10% carbamide peroxide, Opalescence (Opa.Ultradent), used as control. At baseline (Bas), 6 indentations (Vickers) were made on each of the 30 enamel surfaces, under load of 100g for 30s. Bleaching procedure was performed 1-hour a day (30'a.m. and 30'p.m), except for Opa (8-hours a day) for 2 weeks. After bleaching diary time, the specimens were rinsed and maintained in fresh artificial saliva, changed daily. After being bleached (AfB), another 6 indentations (Vickers) were made on each of the 30 enamel surfaces. A t-test compared each bleaching agent pre and post treatment. Results: Per.Bas= 316.1(±34)A, Per.AfB=323.8(±38)A; DW5.Bas=324.6(±35)A, DW5.AfB=350.8(±42)B; DW7.Bas=325.7(±42)A, DW7.AfB=331.3(±45)A; Whi.Bas=332.2(±25)A, Whi.AfB=340,1(±27)A; Opa.Bas=332.1(±33)A, Opa.AfB=331.4(±34)A. Means with the same letter are not significant different at p<0.05. None of the five commercial home bleaching agents reduced the enamel microhardness. Bleaching performed with DayWhite 5.5% gel caused an increase of enamel microhardness. Conclusion: All the five commercial home bleaching agents evaluated in this study showed no adverse effects on enamel microhardness.

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