The differentiation of an adenoma from a metastasis of the adrenal glands is generally believed difficult even by CT or MRI when they are small. In this study, we searched for ultrasonographic distinctions between the two conditions by reviewing 9 cases of the adenomas (8 aldosteronism and one Cushing's syndrome) and 6 cases of the metastasis (5 lung cancers and one breast cancer). In the former, the long axis of the gland passed through a lateral one-third of the tumor in all cases, whereas, in the latter, it passed through the mid-one-third in 5 cases (P<0.01). This conforms to the fact that almost all adrenal adenomas develop in the cortex, while metastases usually occur in the medulla. No differences were noted in terms of their size, contour, internal echo, and the ratio between the long and short axis of a tumor. Although a small pheochromocytoma might be confused with a metastatic lesion since it originates in the medulla, the evaluation of the spatial relationship of a tumor with the adrenal gland itself will help distinguish an adenoma from a metastasis.