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BACKGROUND: The authors reviewed smears from fine-needle aspiration biopsies (FNAB) diagnosed as 'papillary lesions' and correlated the cytologic findings with the final diagnoses at excision. The objective of the current study was to determine the accuracy of FNAB diagnosis of a papillary lesion in distinguishing true papillary from nonpapillary proliferations and to evaluate cytologic criteria for the distinction of papillomas from true papillary malignancies and their cytologic look-alikes. METHODS: The cytopathology database at the New York University Medical Center was searched for women who underwent surgical excision after a breast FNAB diagnosis of a papillary lesion. The FNAB smears and corresponding slides from excisional biopsies were reviewed. The smears were evaluated and graded for the following features: cellularity, architecture, presence of fibrovascular cores, single cells, columnar cells, cellular atypia, myoepithelial cells, foamy histiocytes, and apocrine cells. The F test was used to determine the statistical significance of differences between true benign papillary lesions (papilloma) and adenocarcinomas (in situ and invasive). RESULTS: At the time of excision, 46 (66 %) cases were benign (23 solitary intraductal papillomas, 6 intraductal papillomatosis, 11 examples of fibrocystic change, and 6 fibroadenomas) and 24 (34 %) were malignant (1 low-grade phyllodes tumor [PT], 23 ductal in situ and invasive carcinomas). Of the 23 carcinomas, 3 (13 %) were classified as benign papillary lesions on FNAB and 19 (86 %) were classified as either atypical or suspicious. One case of low- grade PT originally was classified as benign on FNAB. There were four false-negative diagnoses; two were due to sampling and two to interpretative errors. A portion of the lesions classified as papillary were fibroadenomas and examples of fibrocystic change on excision and all of these were correctly classified as benign on FNAB. Of the histologically proven papillomas, 62% were correctly classified as benign on FNAB and none were designated as being positive for malignancy. Statistically significant features of distinction between papillomas and carcinomas included cellularity (P = 0.016), cellular atypia (P = 0.0053), and the presence of cytologically bland columnar cells (P = 0.04). Low-grade ductal carcinoma in situ (cribriform and micropapillary types) and tubular carcinoma represented the most difficult differential diagnostic problems. CONCLUSIONS: A significant portion of lesions displaying a papillary pattern on FNAB are nonpapillary on follow-up. Among benign processes, fibrocystic change and fibroadenoma may closely simulate papilloma on cytology. However, in spite of the overlapping features of true papillary lesions and their cytologic look-alikes, the majority can be classified accurately into benign or atypical (and above) categories by FNAB. Lesions that fall short of a definitive benign diagnosis should be placed into an indeterminate category. This approach will guide the surgeon to provide better patient management