The Weiss scoring system encompassing 9 histological criteria for tumor grading the Weiss index (WI) and modified Weiss index (MWI), which evaluate the 5 most informative histological criteria with the exception of some inestimable signs has gained the widest acceptance for the morphological diagnosis of adrenocortical carcinoma (ACC). Objective: to comparatively analyze the efficiency of WI and MWI in the diagnosis of ACC. Subjects and methods. Adrenal tumors (ACC and adenoma) from 104 patients were studied. The accuracy of WI and MWI for the diagnosis of ACC and adrenocortical adenomas was comparatively analyzed. Results. The sensitivities of WI and MWI for the diagnosis of ACC were 96.8 and 91.5%, respectively. The difference was statistically insignificant (p=0.2). Conclusion. MWI is less subjective, easier-to-use, and more informative for the differential diagnosis of adenomas and ACC at borderline WI values. However, there is little point in using MWI to diagnose without considering WI as MWI unreached the threshold value (3 scores) more frequently than WI in malignant tumors.