The cytomorphologic features of in situ proliferative breast lesions encompass a spectrum ranging from those of a simple benign lesion to atypical to those indistinguishable from invasive carcinoma. In the majority of cases, however, the features are distinctive and easily recognized, and allow accurate categorization of these lesions on the basis of fine-needle aspiration biopsy. As in tissue sections, recognition of these lesions is based on both cytologic and architectural characteristics. Familiarity with these features is necessary for the appropriate management of cases of in situ proliferative breast disease. Among the proliferative breast lesions, those with no associated atypia represent the majority of cases and can be clinically monitored without the needfor confirmation biopsy. In lesions with associated atypia, however, the cytomorphologic features may overlap with those of low-grade carcinoma, and thus tissue biopsy is required for a definitive diagnosis. The value of ancillary studies to further categorize these lesions is limited.