Background: Laser resurfacing of photodamaged skin has become popular, but questions regarding its safety with regard to the risks of scarring have arisen. Objective: This study was designed to investigate the depth of vaporization and residual thermal necrosis of single-pulse vaporization and multiple passes versus pulse-stacking and multiple passes. The potential significance of operator technique and laser parameters is considered. Methods: Skin samples from surgical excisions were treated by means of a Coherent Ultrapulse carbon dioxide laser at 250 mJ per pulse and 500 mJ per pulse with a 3 mm collimated beam and a repetition rate of 10 Hz. A total of 70 treatment areas were performed. Blinded analysis of the histologic effects of single-pulse, double-pulse, and triple-pulse vaporization after 1 through 10 passes was undertaken. Results: A plateau of vaporization was observed after 3 passes at both 250 and 500 mJ whether single-, double-, or triple-pulse vaporization was used. This plateau occurs at approximately 100 to 250 μm from the skin surface. Thermal necrosis is well controlled only with single-pulse vaporization. There is a direct linear increase in the depth of thermal necrosis both with the number of pulses stacked and the number of passes. Conclusion: Pulsed carbon dioxide laser resurfacing is a safe and self-limited procedure if a pulse width of less than 1 msec is used with single-pulse vaporization and fluences of 3.5 J/cm² and 7.0 J/cm². There appears to be little justification for performing more than 3 or 4 passes. Pulse stacking may significantly increase residual thermal necrosis, thereby increasing the risk of scarring. Operator technique may be significant in avoidance of this occurrence. (J Am Acad Dermatol 1999;40:615-22.)