The purpose of this study was to evaluate the suitability of Q-switched Er:YAG radiation with a pulse duration of approximately 150 ns for caries ablation in dental enamel and dentin. The rate and efficiency of ablation were determined at various laser fluences via perforation of enamel and dentin thin slabs. Peripheral thermal and acoustic damage was evaluated using optical and electron microscopy. Enamel and dentin were ablated with extremely high precision without peripheral thermal damage using these short laser pulses. However, mechanical damage resulted from stress transients produced during the ablative process which caused fracture s in dentin and enamel on the back side of the perforated tissue samples. The thickness of the layer of spallated dentin increased linearly with deposited energy consistent with proposed models. The possibility of acoustic-mechanical damage may limit the maximum single pulse energy that may be deposited when using short pulsed Er:YAG lasers for hard tissue use. This work was supported by NIH/NIDR Grant R29DE12091.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.