Tumor adenosis represents a rare lesion, which most frequent appears as a palpable mass, with more than the usual dimensions. The paper presents a case of tumor adenosis of the mammary gland of a 16 years girl. Macroscopically, the tumor was well delimited, with gray-white surface and a nodular aspect on section. The tissue was processed by routine technique and stained with hematoxylin-eosine (H&E). There were also performed immunohistochemical (Actin SM, CK, S100) and special stainings (silver impregnation). Microscopically, there were observed several histologic types of adenosis, the most frequent being sclerosing adenosis. Through silver impregnation it was observed that the basement membrane, which surrounded the tubes, was intact. Immunohistochemically, there were evidentiated the epithelial and myoepithelial components of the tubes. The presence of these elements led through the diagnosis of tumor adenosis. The importance of recognition of the tumor adenosis is found in the possibility of wrong interpretation of this with tubular invasive carcinoma, these two entities being often mistaken, mainly at the frozen-section examination.