MR studies were performed in 36 patients with proven renal cell carcinoma. In 27 patients in whom a radical nephrectomy was performed, MR studies were reviewed and compared with CT and pathologic findings. Renal cell carcinomas had a varied MR signal with the most common appearance being a mass with an intensity intermediate between the renal cortex and the medulla on T1-weighted images and hyperintense on T2-weighted images. MR was similar to CT in staging renal cell carcinomas (74% for MR vs 67% for CT). Neither CT nor MR was reliable in differentiating stage I from stage II lesions. MR clearly showed venous invasion without the use of IV contrast medium and was superior to CT in differentiating lymphadenopathy from small vascular structures. Because of the limited availability, longer imaging time, and more stringent patient requirements (patients with pacemakers, intracranial aneurysm clips, life-supporting systems, and severe claustrophobia need to be excluded), the authors recommend MR only for patients with known contraindications to iodinated contrast medium, patients with prior suboptimal bolus-contrast CT studies, or patients in whom the CT findings are equivocal.