We report 62 cases of invasive micropapillary carcinoma of the breast characterized by delicate pseudopapillary structures lacking a fibrovascular core and by tubuloalveolar structures freely floating in clear, empty spaces. All patients but 1 were women (median age, 57 years; range, 25-89 years). Tumor size ranged from 0.7 to 10 cm (median, 2.8 cm); 54 (87%) were grade 3. Psammoma bodies were identified in 29 (47%). Focal to massive lymphatic permeation was present in 39 (63%). Architectural features were retained in the node metastases, dermal lymphatics, and recurrences. Fifty-six patients (90%) had metastatic axillary nodes: 18 tumors were estrogen receptor-positive (32%); 11 were progesterone receptor-positive (20%); HER2/neu was overexpressed in 53 (95%) and p53 in 39 (70%). A peculiar immunoreactivity for MUC1 limited to the cytoplasmic membrane oriented toward the stroma and an absence of immunoreactivity for E-cadherin in the same side of the cytoplasmic membrane indicated inversion of cell polarization and a disturbance in the cell adhesion molecules. Of 41 patients with available follow-up, 29 (71%) had local recurrence (mean, 30 months) and 20 (49%) died of disease. These results underscore the aggressive behavior and poor prognosis of this breast carcinoma variant. Aggressive preoperative neoadjuvant chemotherapy should be considered.