Twenty-six tumours of the mediastinum and hilar regions were examined by magnetic resonance; the technique and initial experience in distinguishing normal from pathological appearances are described. ECG triggering has proved essential, whereas respiratory triggering was found to be unnecessary. Spin-echo images were found to be best for tumour delineation but, for further characterisation, the inversion recovery mode and multi-echo technique were most satisfactory. In general, the space-occupying lesion was seen as well as it would be with CT. The advantages of the new method are the excellent differentiation of vascular structures and the ability to image in multiple planes, whereas CT possesses better resolution which, in some cases, may be of diagnostic advantage.