Thirty-three patients with mediastinal masses were examined by means of MRI to obtain elements for lesion characterization, to evaluate lesion extent, and to assess the relationship of the mass to the surrounding structures, especially the great vessels (accuracy: 84.84%), mediastinal structures (accuracy: 76.78%), and lung parenchyma (accuracy 69.69%). MR data were compared to CT findings; sensitivity, specificity and accuracy were subsequently compared to surgical and histologic findings. The results have shown MRI to be a very reliable method in the evaluation of mediastinal masses.