After reviewing 986 consecutive cases of breast carcinoma from our files, 27 ductal infiltrating cancers showing micropapillary differentiation in invasive areas (MP) were selected. Special immunohistological and ultrastructural techniques were used, and their characteristics compared to those of the Not Otherwise Specified type of carcinomas (NOS). Diagnostic areas of MP were easily identified in H&E sections and were composed of solid or tubular neoplastic cell groups inside a spongy background, where they appeared to swim in aqueous or mucinous material. Neoplastic cells displayed the reverse polarity typical of the papillary phenotype. This was revealed by the detection of acid mucinous rims, lineal deposits of EMA substances, and microvilli in a peripheral position, even in areas where the micropapillae resembled tubules. Histologically, most MP were mixed with N0S, Papillary, or Mucinous patterns, but regardless of the extension of their micropapillary diagnostic component, their tumour size, or their WHO histological grade, two thirds had extensive lymphatic vessel invasion and all the cases presented massive axillary lymph node metastasis. Six of the twelve patients followed died within a mean of 22 months. In conclusion, we propose the recognition of "Invasive Micropapillary Carcinoma of the Breast" as a new special entity with a potentially high degree of aggressiveness.