The advent of mammography screening presents a diagnostic challenge to the cytopathologist as an increasing proportion of breast lesions requiring investigation will be nonpalpable and up to 40% will be accounted for by atypical intraductal hyperplasia and ductal carcinoma in situ, as opposed to previously, when these lesions represented less than 10% of palpable tumors. We studied 133 fine-needle aspirates from breast tumors and found that nuclear morphology, myoepithelial cells, signs of invasion, and degree of cellular dissociation are among the most potent factors discriminating between benign epithelial proliferations, atypical intraductal hyperplasia, ductal carcinoma in situ, and invasive carcinoma. Diagn. Cytopathol. 2004;31:207–215. © 2004 Wiley-Liss, Inc.