To investigate the correlation between the neuropathologic findings after laser-induced interstitial thermotherapy (LITT) and MRI temperature maps during laser irradiation, a total of six pig brains from cadavers were treated with a Nd:YAG laser (lambda = 1,064 nm, 3.8 W) for 15 minutes. For MRI monitoring, we used a phase-sensitive two-dimensional (2D) fast low-angle shot (FLASH) sequence on a 1.5-T Magnetom SP. Temperature maps were acquired every minute (accuracy, <1.5 degrees C). Histopathologic methods were selected (hematoxylin and eosin stain, Masson's trichrome, Bodian silver impregnation) to demonstrate the zonal architecture of LITT lesions in ex vivo specimens. They showed extensive destruction of the nervous tissue constituents, vascular changes, and lysis of erythrocytes near the track of the laser, a transitional zone, and an 1.5-mm broad margin with edema-like perinuclear vacuolization. No immunoreactivity of glial fibrillary acidic protein (GFAP) could be visualized inside the lesion. In a semiquantitative evaluation, the lesion sizes were measured microscopically (mean diameter = 12.8 mm) and their margins could be defined at temperature zones of 60 to 65 degrees C on the MRI maps.