The purpose of this in vitro study was to determine the effects of two 10% carbamide peroxide bleaching solutions on the hardness of enamel from the surface to the dentinoenamel junction. Fifteen anterior teeth wre sectioned faciolingually, and their roots were removed. Hlf of each crown was bleached and half served as its own control. Specimens were bleached with two commercially available 10% carbamide peroxide solutions (Proxigel and gly-Oxide) for 24 1-hour periods. A visible color change occurred. Teeth were embedded in acrylic resin rings, sanded with progressively finer silicon carbide papers, and polished. The polishing process removed approximately 800 jm of the cut tooth surfact. Specimens were tested with ah microhardness tester and a Knoop diamond under a 35-g load; measurements were taken along the cut surface from the outer enamel surface to the dentinoenamel junction. Matching depths from control and test specimens were compared. For teeth treated with Gly-Oxide, no statistically significant changes in enamel hardness were evident at 50 um or deeper. A statistically significant decrease in hardness was noted in the outer 25 um or deeper. A statistically significant decrease in hardness was noted in the outer 25 um of the enamel surface. It is unknown whether this decrease would have an effect clinically because the remineralizing effects of saliva should resolve any surface change.