Magnetic resonance (MR) imaging was performed in 10 patients with biopsy proven superior sulcus carcinomas to determine the extent of local tumor invasion. Chest wall invasion or extension into the base of the neck was demonstrated in five patients. This finding was facilitated by the contrast between the bright signal intensity of tumor and the low signal intensity of muscle on T2-weighted images. In three patients, MR clearly depicted direct invasion of the mediastinum. This finding was facilitated on the MR image because of inherent contrast between the mediastinal fat and tumor. Coronal and sagittal images showed the relationship of tumor to the subclavian artery and brachial plexus in all cases. Encasement of the artery was confirmed in two cases and brachial plexus involvement in three. However, MR failed to detect evidence of rib destruction in five patients in whom rib involvement was confirmed by other studies. Magnetic resonance appears to be a useful imaging modality in determining the extent of local disease and, therefore, the selection of patients for surgical resection. Multiplanar imaging and contrast between tumor and muscle and tumor and fat allow assessment of invasion of the mediastinum and base of the neck, subclavian artery, and brachial plexus.