Examination of two adenocystic carcinomas of the mammary gland has revealed main criteria for the morphological diagnosis of this malignant tumor. This is characterized by the presence of cribrose structures that are formed by multiple pseudocysts filled with amorphous substance. Clusters of small, monomorphic, cubic cells with round or oval nuclei form abundant islets at whose expense a characteristic sieveform pattern develops. The myoepithelial cells are frequently detected in the tumor. A favorable prognosis is seen in adenocystic carcinoma of the mammary gland. The two patients are alive having no signs of disease progression in their 9- and 51-month follow-up, respectively.