Fine-needle aspiration biopsy (FNAB) of invasive lobular carcinoma (ILC) is associated with notoriously high rates of false negative and equivocal diagnoses. To identify causative factors, we reviewed the cytologic features of presurgical FNAB smears of ILC and correlated the cytologic findings with the number of passes, tumor size, mammographic findings, and the histologic characteristics of the tumor. Smear cellularity, presence of single intact epithelial cells, nuclear size, nuclear atypia, palpability of the tumor, and histologic type of ILC (classic versus nonclassic) were statistically significant in establishing an unequivocally positive diagnosis. We also found that the cytologic cellularity of the lesion does not reflect the actual cellularity of the tumor but instead is an indicator of the architectural arrangement of the neoplastic cells; tumors that form epithelial cell groups, such as in nonclassic ILC, tend to yield more cellular aspirates that are diagnostic for carcinoma. In contrast, classic ILC, in which single neoplastic cells are embedded in fibrous stroma, is more likely to yield a paucicellular smear with subtle atypia and rare single intact epithelial cells. As such, an inconclusive diagnosis in a certain percentage of classic ILC cases may be unavoidable. Diagn. Cytopathol. 2004;31:87–93. © 2004 Wiley-Liss, Inc.