Differentiation between benign and malignant tumors of the adrenal cortex was studied by means of seven histologic parameters. Each separate criterion was significantly different in two groups, one consisting of patients without metastases within 10 years after operation and one of patients with metastatic tumors. Discrimination on the basis of single parameters in individual patients, however, was of little value. When a histologic index was calculated using the same parameters but after "weighing," a much better discrimination was obtained. The histologic index and, to a lesser degree, the mitotic activity seems to be related to the survival time of the patients with adrenal cortical carcinoma. The authors conclude that the calculation of a histologic index based on a summation of different parameters makes it possible to differentiate between malignant and benign adrenal cortical tumors. As single parameters, tumor weight and mitotic activity have the highest discriminating value.