To describe the cytologic features of invasive micropapillary carcinoma of the breast, a distinct, unusual variant of infiltrating duct carcinoma characterized histologically by pseudopapillary structures lacking fibrovascular cores and tubuloalveolar structures floating freely within clear spaces separated by a delicate fibrocollagenous stroma and characterized clinically by prominent lymphotropism and an aggressive clinical course. We reviewed the clinical, cytologic, histologic, immunohistochemical and flow cytometric features in 11 cases of invasive micropapillary carcinoma of the breast diagnosed preoperatively by fine needle aspiration. All patients were women, ranging in age from 31 to 83 years. A preoperative diagnosis of malignancy was made in all cases. Tumor size ranged from 1.2 to 5.5 cm. Ten patients had lymph node metastases. Cytology was characterized by a "dual" pattern formed by round or angulated, three-dimensional, cohesive clusters of neoplastic cells with pseudopapillary configuration and two-dimensional, dyscohesive aggregates and single cells with high grade nuclei and intact cytoplasm.