The aims of this study were, firstly, to monitor temperature with magnetic resonance (MR) during laser ablations performed in pig livers in vivo in a low-field open scanner (0.23T) and, secondly, to study the feasibility of lesion size prediction. Spin-echo (SE) images of 29 sec acquired during laser applications allowed calculation of temperature maps using T1 and M(0) temperature sensitivity. Temperature was also measured with thermocouples. Images of prediction of tissue damage were calculated using temperature maps and Arrhenius model. T2W sequences were acquired after the ablations. Animals were sacrificed immediately. Lesions were photographed macroscopically. Lesion surfaces were measured and compared in T2W images, temperature images, damage prediction images, and macroscopic pictures. A correlation exists between temperature measured with MR and with thermocouples (rho = 0.878; P < 0.001, Spearman test). Mean surface of predicted damaged tissue is consistent with mean early necrosis measured in macroscopic pictures. Early T2W images underestimate mean necrosis size. J. Magn. Reson. Imaging 2001;13:42-49.