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The ablation rate and thermal damage in skin produced by a superpulsed CO 2 laser operating at pulse repetition rates between 1 and 900 Hz were measured. When delivering a fixed number of pulses (20 or 30) of equal energy, a 55-60% increase in the amount of tissue ablated was observed when the pulse repetition rate rose from 10 to 200 Hz. At pulse repetition rates greater than 200 Hz no further increase was seen. Under identical conditions, an 80% increase in the zone of thermal damage was observed when the pulse repetition rate was increased from 1 to 60 Hz. The large increases in tissue ablation and tissue damage may indicate the existence of a layer of mixed-phase (ie., liquid and vapor) or metastable liquid which can store significant amounts of thermal energy between pulses. The data suggest that CO 2 lasers should be operated at relatively low repetition rates for optimal performance.