Ten volunteers with a normal mediastinum were investigated by magnetic resonance using a 0.5 Tesla CGR imager with a supraconducting magnet. The reconstruction matrix consisted of a 256 X 256 grating for a field of the order of 420 mm, with a spatial resolution of 1.6 X 1.6 mm2. The sections, balanced in T1, were performed in synchronization with the ECG. The successive sagittal sections were correlated with sagittal sections made on a single embalmed frozen subject. The MRI and anatomic sections were made at 5 mm intervals and located in relation to the median sagittal plane of the mediastinum. Examples of structural variations, malformations or tumours studied in sagittal sections, and taken from investigation of over 170 patients, demonstrate the importance of this investigational technique.