The aims of this study were to calibrate the collection depth for RMS and, using the calibration, to evaluate the in-depth phase transformation induced in a dental 3Y-TZP ceramic by hydrothermal aging, simulating LTD. Objectives 10x and 80x affording spatial resolutions of 3 and 1 mu m, respectively, were used. Probe size was controlled by the confocal pinhole aperture. The collection depth of each probe (objective pinhole combination) was calibrated on a wedge-shaped pure tetragonal zirconia specimen. Pure tetragonal samples of a dental 3Y-TZP were aged in vitro in an autoclave, in Ringer solution, at 130 degrees C, under 0.6 MPa, for 10, 15, 20, 25, and 90 h. Two specimens were used for each time interval and two unexposed specimens served as controls. Monoclinic volume fraction (Vfm) was determined by X-ray diffraction and by in-depth probing with RCMS. Probes based on 10x objective penetrated up to 68 mu m, whereas those based on the 80x were limited to 5 mu m. Aging induced significant increase in Vfm, ranging from 30% after 10 h to 80% after 90 h. After 90 h aging, the 80% Vfm extended to at least 20 mu m from the surface. A maximum Vfm was identified at 2.5 mu m subsurface depth in 10, 15, and 20 h aged specimens.