Adrenocortical carcinoma is an unusual tumor that tends to be larger than 100 g at diagnosis. Malignant adrenocortical tumors of less than 50 g have been reported rarely. We studied a small adrenocortical carcinoma that weighed 40 g and metastasized 3 1/2 years following bilateral adrenalectomy. The malignancy of adrenocortical tumors is frequently difficult to determine. Two recent studies have proposed histologic criteria for determining the malignancy of adrenal cortical tumors. Since the tumor we studied was initially judged to be benign, we sought to determine whether an extensive evaluation of this tumor using these recent criteria could have identified the tumor's potential for metastasizing. Careful application of these criteria indicated that the tumor was either a borderline lesion or a definitely malignant lesion. This emphasizes the need to approach systematically all adrenal neoplasms with regard to determining their malignant potential. Documenting small adrenocortical carcinomas also should help in setting criteria for evaluating adrenal masses discovered incidentally by sensitive radiologic methods.