Abstract: Infiltrating micropapillary carcinomas (IMPC) of breast are highly angioinvasive tumors with poor prognosis. This study is based on the observation that a similar micropapillary pattern is also observed in mucinous carcinomas of breast. About 102 mucinous carcinomas were evaluated for the presence and impact of this micropapillary pattern on the clinical behavior. Of these, 68 were mucinous carcinomas with a micropapillary pattern (MUMPC), 20 had MUMPC mixed with an infiltrating duct carcinoma component, two were solid variants of papillary carcinoma with mucin (SVPCMU), five had collision of the MUMPC and SVPCMU patterns and seven were mucinous carcinomas with signet ring cells (MUS). The factors negatively affecting overall survival (OAS) and disease-free survival (DFS) included the histological type of mucinous carcinoma, nodal metastases, an irregular tumor border, <50% mucin and an IMPC type of local recurrence or metastases. In the multivariate analysis, the histologic type of mucinous carcinoma and an irregular tumor border were most significant for OAS and DFS. Thus, 86% of mucinous carcinomas in this study were mucinous variants of the angioinvasive infiltrating micropapillary carcinomas. These tumors can produce IMPC type of metastases and thus should be treated aggressively.