A method of marking human enamel so that reduction in enamel thickness by etching, bonding, and wear could be measured to within a range of -2.5 to +3.5 microns for the impression accuracy and to within a range of -1.5 to +3.8 microns for the measurement error, was devised and used to measure the effects of the above factors on enamel. For five unetched surfaces studied in vivo, the 85-day loss of enamel averaged 1.6 microns. For twenty teeth etched and studied in vivo, the etch removed 9.9 microns of enamel and the subsequent loss over 85 days exceeded normal wear by 3.0 microns. The etch produces microporosities up to 50 microns in depth; therefore, the remaining 40 to 45 microns must be filled rather than worn smooth. For twelve teeth subjected to etching, bonding, bracket removal, and "clean up," enamel removed averaged 55.6 microns or approximately the entire thickness of the etch. The surface immediately following the "clean up" and 2 months after "clean up" were clinically and microscopically comparable to an untouched enamel surface.