Differential diagnosis of adrenocortical cancer (ACC) and cortical adenoma presents certain difficulties since there is no specific histological criterion allowing to distinguish tumors of the adrenal cortex with malignant clinical course. Currently there are offered several systems, and the most widely spread have the index Weiss (IW) and the modified index Weiss (MIW). The accuracy of one or another of the proposed systems remains a matter of debate. There was analyzed own experience on the use of IW and MIW in the diagnosis of 91 cases of the ACC and 13 cases of cortex adenomas of the size at least 5 cm. For the diagnosis of large adenomas sensitivity IW was 77%, MIW--100%. For the diagnosis of metastatic and non-metastatic ACC--100% and 97%, 100% and 86%, respectively (p > 0.05). In multivariate analysis of life expectancy of patients the definition of IW and MIW had a prognostic significance. MIW was less subjective, more simple and convenient to be used and it showed a great informative value at the reclassification of certain "adenomas" into ACC. However to use it on their own, without IW, was impractical as MIW had wider gray area and did not reach the threshold value in some cases of ACC. For the diagnosis of tumors of the adrenal cortex IW remains a standard; when a value was equal of 2 or in cases of doubt it was necessary to calculate MIW as well.