Tooth bleaching continues to develop in popularity.  Vital tooth bleaching is administered in a variety of forms including trays, strips and paint on high concentration treatments.  The concentrations and conditions of bleaching vary considerably between these treatment forms.    Objectives: This study compared physical and micromorphological surface changes associated with exposure of human enamel specimens to a cycle of chronic bleaching with daily hydrogen peroxide bleaching strips (designed for daily application) in vitro.  Methods: Extracted human premolars were prepared in Durabase blocks and measured before treatments for surface microhardness (Vickers 500 g load).  Bleach-cycled teeth were bleached with 6.5 % hydrogen peroxide bleaching strips (Crest Whitestrips^® Daily Multicare^TM) 4x/day for a total of 720 treatment cycles – taking 190 treatment days.  The bleached group also received bid daily treatment with Crest Regular dentifrice as a 25 % slurry for 5 minutes per day which was followed by a brief surface brushing under tap water.  A cycled-control group received dentifrice treatments only while a third group was saved as a non treated control.  During the cycling, groups of teeth from cycled groups were removed at treatment # 120, 240, 360 for histological examination.  Here treatment teeth were compared for final surface microhardness and also surface roughness measured with Taylor Hobson profilometer.  Results:   VHN measurements for enamel at T=720 treatments:  Bleach = 331±15 -nsd- Cycle/Control = 322±12 -nsd- Untreated = 327±15.  Surface roughness T=720:  Bleach = 0.026±0.004 -nsd- Cycle/Control = 0.032±0.010 -p<0.001- Untreated = 0.012±0.001 [Students t].  Conclusions:  Chronic bleaching representing up to two years daily use had no effects on hardness of enamel surfaces as compared to cycled non-bleached controls.  The act of cycling through 190 days of salivary/paste treatment including brushing resulted in slight roughening of surfaces as compared to non-cycled non treated controls. 

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