The usefulness of MR spectroscopic imaging for discriminating between lipid and water was applied to the in vivo differentiation of adrenal adenomas from carcinomas. By using the Dixon sequence in 20 patients, the lipid content of 22 adrenal tumors larger than 15 mm was determined. The mean percentage of lipid in 15 adenomas was 13.4% (standard deviation, 8%), compared with 3.5% lipid (standard deviation, 2%) in seven carcinomas. Only one lesion would have been misclassified on the basis of in vivo measurements of lipid content. After surgery, in vitro MR spectroscopy was used to determine the percentage of lipid in excised samples of nine of the 22 tumors. These in vitro measurements confirmed the in vivo results on lesions larger than 20 mm in diameter. Respiratory artifacts appeared to decrease the accuracy of in vivo measurements in smaller lesions. In vivo MR spectroscopic imaging of adrenal tumors appears to be useful for differentiating between adrenal carcinomas and adenomas.