The detection of early, well-differentiated breast carcinoma is increasing because of mammographic screening. Because fine-needle aspiration cytology is often used as an adjunctive diagnostic tool with mammography, the authors defined the cytologic criteria for the diagnosis of this early breast carcinoma. Aspirates from 24 cases of biopsy-confirmed tubular carcinoma were studied. The majority of these carcinomas were detected by mammography and were nonpalpable masses. Mammographically, the lesions were .3-1.5 cm and were often described as ill-defined, spiculated densities. The cytologic diagnoses in these 24 cases were as follows: negative (4), fibrocystic changes (1), atypia (12), malignant (3), and fibroadenomas (4), (including 2 fibroadenomata with atypia and 1 suspicious for carcinoma). The aspiration cytology in all cases were reviewed, and the following characteristics were found in the 20 adequate smears: mild to moderate atypia, increased cellularity, angular epithelial clusters, and single epithelial cells ranging from few to numerous. Myoepithelial cells were prominent in 7 of 20 smears. The authors compared these features with aspiration cytology of 10 fibroadenomas, as this was a frequently suggested diagnosis. The fibroadenomas showed no significant atypia, minimal angular epithelial clusters, and rare single epithelial cells. Myoepithelial cells were prominent in all fibroadenomas. The authors concluded that the presence of angular epithelial groups and single epithelial cells, along with nuclear atypia, should warrant consideration of the diagnosis of tubular carcinoma.