The effect of hamster cheek pouch temperature upon the microvascular injury induced by pulsed laser dye radiation was studied. An appropriate wavelength (577 nm) and pulse width (300 nsec) were delivered by a flashlamp-pumped dye laser to induce selective vascular damage, as previously described. With increasing pulse energies, transient slowing of blood flow, stasis with brown discoloration of blood, and hemorrhage were observed. Cooling the cheek pouch to 8 degrees before irradiation increased the laser exposure threshold for induction of such injury. Measurement of the exposure thresholds for hemorrhage at different temperatures suggests that microvessels are heated to between 90 and 100 degrees, before hemorrhage occurs. These data are consistent with microvaporization as a major event in the sequence leading to hemorrhage, but do not exclude other possible mechanisms.