Contrast-enhanced dynamic incremented CT scans in 37 patients with 44 small adrenal masses (28 benign and 16 malignant) were reviewed by two observers unaware of the histologic diagnosis to determine if applying morphologic criteria could help differentiate small benign adrenal masses from malignant adrenal masses. Only lesions smaller than 5 cm with diagnoses confirmed by histology (12 masses) or follow-up (32 masses) were included. Features evaluated to suggest a benign diagnosis were homogeneous low attenuation, possibly with punctate contrast enhancement; an enlarged gland (adrenal configuration maintained); a thin or absent rim; round or oval shape with sharp margins; and diffusely homogeneous attenuation about equal to or greater than that of muscle. Features studied to suggest a malignant diagnosis were a thick enhancing rim, invasion of adjacent structures, irregular or poorly defined margins, and inhomogeneous attenuation. Both observers' diagnoses of benign vs malignant lesions with CT criteria were highly statistically significant. The positive predictive value of a benign diagnosis was 100% for both observers and of a malignant diagnosis was 82% and 62% for the two observers. Evaluated singly, all but three diagnostic criteria were statistically significant in differentiating lesions for both observers; the other three criteria were present in a smaller percentage of patients, but nevertheless had positive predictive values for benignancy of 89-100%. We conclude that experienced observers who use CT criteria can often discriminate accurately between benign and malignant small adrenal masses and, in particular, minimize the number of false-negative diagnoses of adrenal metastases. If these results are confirmed and refined by prospective studies, aggressive diagnostic evaluation can be eliminated in some patients with benign adrenal lesions.