Assuming that the irradiance of the laser beam is much greater than the threshold for ablation, a simple expression is obtained for the depth of ablation crater as a function of incident fluence. It is expected to be valid even where mechanisms other than thermal dominate the ablation process. A high level of irradiance is necessary to avoid excessive heating of adjacent tissue by heat diffusion. Contrary to expectations based on non-ablative heating, short pulses are not necessarily more effective than longer pulses in minimizing heat diffusion to adjacent tissue. Peripheral heating can further be reduced by using a wavelength with high absorption (short penetration depth) in tissue. Under appropriate conditions of wavelength and irradiance, clean, "cold" cuts are possible with the thermal mechanism of ablation. © 1989 SPIE.