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 reccur- rence. 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. © 1990 S. Karger AG, Basel.