Bleaching of teeth by "in-office" or "home" bleaching techniques are popular methods of whitening teeth. However, bleaching may reduce the surface hardness of enamel and dentin. The purpose of this study was to evaluate (1) the effect of different concentrations of 2 "in-office bleaching" and 2 "home bleaching" agents applied for different time periods on the hardness of enamel and dentin and (2) the effect of subsequent immersion in a low-concentration fluoride solution on the hardness of bleached enamel and dentin. The enamel and dentin of 12 extracted intact human molar teeth were sectioned lengthwise, ground, polished, embedded in acrylic resin and divided into 4 groups each (n=12). An area of approximately 5 x 5 mm of enamel and dentin tested for Knoop hardness number (KHN; kg/mm 2) at a load of 100 g for 20 seconds (baseline). The specimens were stored in distilled water for 1 hour and the microhardness testing repeated as a control group. The groups were bleached as follows: Group OX and Group OQ were bleached "in office" with Opalescence Xtra (35% hydrogen peroxide) and Opalescence Quick (35% carbamide peroxide), respectively, for 5, 15, or 35 minutes and retested for KHN at the end of each time period. "Home bleaching" products Opalescence F (15% carbamide peroxide) and Opalescence (10% carbamide peroxide) were applied in 14-hour applications at 24-hour intervals to Groups OF and O, respectively, which were then tested for KHN. Specimens were immersed in 0.05% fluoride solution (Meridol) for 5 minutes and retested for KHN. The hardness values were analyzed by 2-way ANOVA and Scheffé post hoc test (α=.05). Comparisons of KHN between each time and the baseline measurement for each group were of interest. Significant decreases in KHN of enamel and dentin were found after bleaching for all test groups, dependent on the accumulated bleaching time. Group OX showed a 25% KHN reduction for enamel and 22% for dentin after 35 minutes bleaching (P<.0001). Group OQ showed a 13% KHN reduction (P<.0001) for enamel and 10% for dentin after 35 minutes (P<.005). Group OF showed a KHN reduction of 14% for enamel (P<.05) and 9% for dentin (P<.0001) after 14 hours bleaching, and Group O showed an 18% reduction in enamel (P<.0001) and 13% in dentin (P<.0001) for the same period. Fluoridation completely restored the softened dental tissues. The "in-office" bleaching technique reduced the hardness significantly more than the "home" bleaching technique. Low-concentration fluoride mouth rinse (Meridol) restored the softened dental tissues.