In this preliminary study, the use of real-time ultrasonography to visualize the effects of acute interstitial Nd:YAG laser irradiation was investigated in the normal pig brain. In six pigs, a craniotomy was performed. In the frontal or temporal lobe, a thermal laser lesion was made using a 600-micron-diameter optical fiber at powers of 1 W, 2 W, and 4 W with exposure times of 5 min and 10 min. Ten to thirty minutes after laser irradiation, the pigs were sacrificed. Ultrasound imaging was performed before, during, and after laser irradiation. During laser irradiation, a clear hyperechogenic area was observed around the fiber tip. The onset of the changes and the extent of the lesion were dependent on the power and exposure time. Histologic examination showed thermal lesions consisting of coagulation necrosis and edema. The size of the lesions correlated well with size on ultrasound imaging. The maximal lesion dimension was 12 mm in diameter (4 W for 5 min). In conclusion, within the limitations of this experimental setup, it is feasible to visualize interstitial laser-induced lesions in the brain by ultrasonography. This method is safe and simple and may be helpful in future applications of interstitial thermotherapy in brain tissue.