The purpose of this study was to investigate the potential value of i.v. gadolinium-diethylenetriamine penta-acetic acid (Gd-DTPA) applied before MRI-guided laser-induced interstitial thermotherapy (LITT) of brain tumors without original enhancement, especially in defining total lesion size during therapy. MRI-guided LITT was performed on two patients with astrocytoma WHO II. For both patients, Gd-DTPA was administered intravenously after a first irradiation period and LITT was continued after pulling back the light guide to coagulate the upper parts of the tumor. In both patients, the whole irreversible damaged zone of the second irradiation period after Gd-DTPA showed an intense increase of signal intensity. The spatial expansion correlated with the diameter of an enhancing rim after Gd-DTPA on follow-up studies. Our preliminary results indicate that the application of Gd-DTPA before MRI-guided LITT may be of value in defining exactly the size of the irreversible damaged zone during therapy in nonenhancing brain tumors.