Since thymectomy is nearly always indicated as a possible treatment of myasthenia gravis, we examined with Magnetic Resonance (MRI) and Computed Tomography (CT) 22 patients (21-62 years old) to identify the best methodological approach. MR images were obtained with a 1.5 T superconducting unit with slice thickness ranging from 6 to 8 mm and an interslice gap of 0.6 and 0.8 mm. Spin-echo images were acquired with repetition time (TR) < 700 ms and echo time (TE) of 20 ms and T2-weighted images with TR > 1800 ms and TE of 80 ms. The sections were obtained, with cardiac gating, on transverse and sagittal planes. CT was performed with contiguous 5-mm slice thickness, after intravenous bolus injection of contrast medium. All the patients underwent surgery of anterior mediastinum and histologic diagnosis was made. Both CT and MRI correctly identified the patterns of normal thymus or hyperplasia not associated with gland enlargement, the only two cases of hyperplasia with thymic enlargement and clearly demonstrated thymomas. MRI appears to be more accurate in the evaluation of the relationship between thymus and contiguous structures. If pericardial infiltration is suspected, sagittal MR scans yield accurate information on tumor spread. We recommend MRI of anterior mediastinum to rule out the presence of a thymoma and the possible involvement of contiguous structures.