Objective — to analyze the clinical and pathomorphological characteristics of types A and B mucinous breast cancer (BC) and micropapillary carcinoma with obvious mucus production. Subject and methods. The paper analyzes the results of examination and treatment in 38 patients. Early-stage (I and IIA) BC was diagnosed in 81.6% of cases in the study. The patients’ mean age was 60.2±7.9 years (range 33—78). The distribution of the patients according to the stages of BC was as follows: 18 (47.4%) patients with Stage I BC, 13 (34.2%) with IIA stage, 3 (7.9%) with IIB stage, 2 (5.3%) with IIIA stage, 1 (2.6%) with IIIB stage, 1 (2.6%) with IIIC stage. The lymph node status was pN0 in 33 cases with no lymph node metastases in mucinous BC and pN1-3 in 5 cases with invasive micropapillary carcinoma (IMPC) of the breast. The multicentricity of tumor nodules was diagnosed in 4 (10.5%) cases; the number of tumor nodules was from 2 to 3 in the breast, as well as in all cases in mucinous BC. The distribution of the patients according to the molecular biological type of a tumor was as follows: luminal A tumor in 21 (58.3%) cases, luminal B Her2-negative cancer in 11 (30.5%), luminal B Her2-positive tumor in 3 (8, 3%), and triple negative BC in 1 (2.9%). There was a triple negative BC type in 1 case of IMPC of the breast, a luminal B Her2-positive type in 3 cases of mucinous BC, and luminal types of the tumor were diagnosed in the other cases. Results. Lymph node metastases were found in 5 (13.2%) cases. In this investigation, all the patients with lymph node metastases had one type of cancer — invasive mucinous micropapillary carcinoma. At the same time, mucinous micropapillary carcinoma was confirmed by biopsy only in 1 female patient. During the follow-up, 38 patients with mucinous and mucinous micropapillary BC were found to have distant metastases in none of the groups. The investigation revealed the following relationship of lymph node metastatic spread: the metastases were diagnosed as T1 in 10% of cases, T2 in 15.4%, and T3 in 25%. Conclusion. The task of a pathologist is to identify and correctly differentiate invasive micropapillary carcinoma with obvious mucus production from simple mucinous carcinomas types A and B in the preoperative period of the examination. So, only a study of sentinel lymph nodes can be performed in only simple mucinous breast carcinoma; axillary lymphadenectomy should be carried out in micropapillary carcinoma with obvious mucus production. Treatment policy for invasive micropapillary BC with obvious mucus production should be more aggressive, while that for mucinous cancer is minimal.