With the use of fine-needle aspiration biopsy to evaluate non-palpable breast lesions, an increasing number of cases of ductal carcinoma in situ (DCIS) are encountered. The authors previously demonstrated that it is not possible to distinguish definitively between DCIS and invasive ductal carcinoma on fine-needle aspiration biopsy. To determine whether DCIS could be separated from atypical ductal hyperplasia (ADH), the authors identified all cases of exclusive DCIS or ADH with fine-needle aspiration biopsy before surgery. Sixteen cases of ADH and 19 cases of DCIS were identified. Of these, 12 in each category were sufficiently cellular to allow evaluation of architectural and cytologic features. Cases of ADH were more likely to be diagnosed as negative or atypical (11 of 12); in contrast, DCIS was more likely to be designated as suspicious or positive (9 of 12). Architectural and cytologic features characteristic of ADH included cells arranged in flat cohesive sheets, distinct cell borders, and myoepithelial cells. Those features characteristic of DCIS were single cells representing more than 10% of atypical cells, cellular dyshesion, and inflammatory background, coarsely granular chromatin, and nuclear pleomorphism. Many other features were not useful in separating ADH from DCIS. Based on this small series, it appears that the distinction between some cases of DCIS and ADH may be possible on fine-needle aspiration biopsy.