Introduction: Today bleaching is a routine noninvasive alternative for treatment of discolored teeth. The concern has been raised whether or not oxidizing reaction during this process, might endanger integrity of the teeth and raise the risk of caries formation. The aim of this study was to determine whether conventional or laser activated bleaching predispose teeth to develop caries or not.Methods:Sixty human molars were mounted on acrylic cylinders and Knoop microhardness (KHN) and DIAGNOdent (DD) values of them were recorded. They were divided into four experimental groups; G1)conventional bleaching with 40% Hydrogen peroxide gel G2) Diode laser assisted bleaching with same gel. G3)Nd;YAG laser assisted bleaching with the same gel. G4)control group. After bleaching, all samples were subjected to a three day pH cycling regimen and then, KHN and DD values were measured.Results:All groups had significant reduction in KHN values. . It seems that there is no statistically meaningful difference between changes in enamel microhardness of sample groups and all groups have changed in a similar amount. Reduction of DD scores were significant in Diode laser and conventional groups, however changes in Nd:YAG laser and control groups were not significant. Changes in DD values have followed a similar pattern among groups, except in G1-G4 and G2-G4 couples. Conventional and Diode laser group had a meaningful difference in reduction of DD values in comparison with control group.Conclusion:It can be concluded that bleaching whether conventional or laser activated method, does not make teeth vulnerable to develop carious lesions
