Thermal injury, a potential mechanism of ultrasound-induced lung hemorrhage, was studied by comparing lesions induced by an infrared laser (a tissue-heating source) with those induced by pulsed ultrasound. A 600-mW continuous-wave CO2 laser (wavelength approximately 10.6 microm) was focused (680-microm beamwidth) on the surface of the lungs of rats for a duration between 10 to 40 s; ultrasound beamwidths were between 310 and 930 microm. After exposure, lungs were examined grossly and then processed for microscopic evaluation. Grossly, lesions induced by laser were somewhat similar to those induced by ultrasound; however, microscopically, they were dissimilar. Grossly, lesions were oval, red to dark red and extended into subjacent tissue to form a cone. The surface was elevated, but the center of the laser-induced lesions was often depressed. Microscopically, the laser-induced injury consisted of coagulation of tissue, cells and fluids, whereas injury induced by ultrasound consisted solely of alveolar hemorrhage. These results suggest that ultrasound-induced lung injury is most likely not caused by a thermal mechanism.