To determine cytomorphological characteristics of proliferative lesions of breast duct epithelium, we reviewed fine-needle aspiration (FNA) smears of 11 cribriform and micropapillary, low nuclear grade, ductal carcinomas in situ (DCIS), nine atypical ductal hyperplasias (ADH), and 10 florid or moderate ductal hyperplasias (DH) without atypia. These breast lesions presented as a palpable mass in 16 patients or were detected by diagnostic imaging in 14 patients. Cytological findings evaluated were cellularity, cell composition, architectural pattern (including presence and shape of intercellular spacing and cell characteristics of epithelial cell groups), nuclear diameter and pleomorphism, chromatin pattern, and number of single epithelial cells. Smears also were evaluated by cytological criteria only, using the scoring system of Masood et al. Based on our study's use of cytological and architectural features combined, the FNA diagnoses were as follows: of the 11 DCIS cases eight were carcinoma and three were inconclusive: proliferative epithelium (ADH v DCIS); of the nine ADH cases two were carcinoma and seven were inconclusive: proliferative epithelium (ADH v DCIS); and of the 10 DH cases four were DH, two were suspicious for carcinoma, and four were inconclusive: proliferative epithelium (DH v ADH). Using the cytological scoring system alone, of the 11 DCIS cases one was classified as carcinoma, five as ADH, and five as DH; of the nine ADH cases one was classified as carcinoma, three as ADH, and five as DH; and of the 10 DH cases four were classified as ADH and six as DH. This study shows that the application of both cytological and architectural criteria to the interpretation of FNA smears is more reliable than cytology alone in the identification of proliferative breast lesions and low-grade carcinoma. However, overlapping features between DH and ADH as well as ADH and low-grade carcinoma exist making separation of some of these lesions difficult. Aspirates of DH and ADH may display many single epithelial cells, mimicking low-grade carcinoma. However, a diagnosis of low-grade carcinoma can be made with confidence if the aspirates are cellular with many single atypical epithelial cells and lack an admixture of benign cellular elements. Architectural and cytological characteristics of proliferative duct epithelium, as evaluated in histological sections, are well represented in aspiration smears and should aid in the identification of these lesions.