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It has been suggested that tissue ablation, collagen shrinkage, and new collagen deposition contribute to the clinical outcome of laser skin resurfacing.To study the effects of fluence and pass number on thermal damage and tissue shrinkage, we performed experiments in an in vivo farm pig model.A CO2 laser was used to treat the flank skin of a farm pig. There were nine treatment sites based on number of passes and fluence per pass. Wound surface areas were measured pretreatment and immediately posttreatment. Biopsies were performed immediately after irradiation.Surface area measurements showed that immediate contraction tended to increase with increasing fluence and pass number up to a maximum of approximately 38% shrinkage, after which the percent contraction remained constant. Thermal damage increased with pass number for low and moderate fluence groups; however, in high fluence groups the thermal damage remained constant with an increasing number of passes.Our results suggest that CO2 laser resurfacing produces immediate tissue contraction and residual thermal damage that is saturable for multiple passes and high fluences. For small fluences, however, there is an almost linear increase in thermal damage and shrinkage with an increasing number of passes.