Abstract: Objective:To investigate immunophenotyping and prognosis of mucinous micropapillary carcinomas (MUMPC) of breast. Methods:Retrospective evaluation was performed on 531 cases with final diagnosis of pure mucinous carcinoma (pMC) of the breast in Tianjin Medical University Cancer Institute and Hospital from January 2003to December 2012, cases with MUMPC and con-ventional pMC without micropapillary patterns were selected. A total of 134 patients with MUMPC were selected as research group, 397 cases of conventional pMC and homochronous 281 cases of invasive micropapillary carcinomas (IMPC) were selected as control groups. Clinicopathologic characteristics, survival analysis and prognosis were compared among three groups. The expression of secret-ed mucins (MUC2, MUC 5AC, and MUC6) and neuroendocrine indicators synaptophysin (Syn) were detected through immunohisto -chemistry in 32MUMPC cases,89conventional pMC cases and 44IMPC cases which randomly selected from the above cases, to iden-tify discriminating immunophenotyping among the three groups. Results: The positive rate of HER- 2 in MUMPC was lower than that in IMPC, but higher than that in conventional pMC ( P=0.001). Similar MUC phenotypes of MUC2, MUC 5AC, and MUC6 were ob-served between MUMPC and conventional pMC. Different phenotypes of MUC 2 were observed between MUMPC and IMPC (P<0.001), no difference was found in MUC5AC and MUC6. In terms of neuroendocrine differentiation, similar synaptophysin phenotypes were detected in MUMPC and IMPC. The positive rate of synaptophysin was higher in MUMPC than in conventional pMC ( P=0.003). Kaplan- Meier analysis results indicated that patients with MUMPC had poorer disease- free survival (DFS) and overall survival (OS) than those with conventional pMC; by contrast, patients with MUMPC had higher survival than patients with IMPC (log-rank for DFS=0.000; log-rank for OS= 0.004). Conclusion:The patterns of immunophenotyping, clinical biological behavior and prognosis indicated that MUMPC exhibit dual phenotypes; these phenotypes displayed similarities and differences compared with conventional pMC and IMPC. Therefore, further studies on therapies for MUMPC should be conducted to avoid insufficient or excessive treatment, and to correctly classify this type of tumor in clinical practice.