BACKGROUND Different concentrations of carbamide peroxide (10 to 22 percent) have been used successfully as bleaching agents, but the adverse effects on enamel microhardness at different times are unknown. METHODS The authors analyzed seven bleaching agents and a placebo. The agents were applied on the surface of human dental fragments for eight hours per day for 42 days and stored in vials containing artificial saliva. Microhardness measurements were performed at baseline, eight hours and seven, 14, 21, 28, 35 and 42 days, as well as at seven and 14 days after treatment had ended. RESULTS Enamel treated with different bleaching agents or a placebo experienced a similar decrease in microhardness values over time, with the exception of fragments exposed to Opalescence PF 20 percent (Ultradent Products, South Jordan, Utah), or OPA20. Until the 49th day, the enamel exposed to OPA20 exhibited the lowest differences from baseline values. After 14 posttreatment days, enamel treated with placebo, Nite White 10 percent Excel (Discus Dental, Los Angeles), Nite White 16 percent Excel and Opalescence 10 percent exhibited the greatest differences from baseline values. An increase in enamel microhardness occurred at the end of the posttreatment period, although baseline values were not reached. CONCLUSIONS Different concentrations of carbamide peroxide agents result in decreases in enamel microhardness. A post-bleaching period in artificial saliva resulted in recovery of baseline microhardness values (one agent) or an increase in values, although baseline values were not reached for these products. Clinical Implications. Higher concentrations of carbamide peroxide containing 0.11 percent ion fluoride can bleach teeth in a shorter period, with fewer hazardous effects on the enamel mineral content.