Laser hyperthermia using Nd-YAG laser was studied experimentally and clinically to treat deep-seated brain tumors. Histological changes, temperature profile, and modification of the blood-brain barrier were studied using cat and rat brains. In a clinical study 5 patients with brain tumors were treated with laser hyperthermia using this computed tomography-stereotactic technique. All tumors of these patients disappeared on computed tomography, and 3 of the 5 patients are still alive without recurrence. It was possible to make optimal lesion and to have accurate peripheral temperature control by using the combination of the Komai stereotactic method and the SLT Nd-YAG laser system. Interstitial laser hyperthermia using this method is easy and safe to use, and it is beneficial to treat deep-seated brain tumors.