In a retrospective study, fine-needle aspirates from 300 nonpalpable breast carcinomas, including 199 invasive tumors and 101 cases of ductal carcinoma in situ, were analyzed to assess the value of 11 features in predicting invasiveness in malignant breast lesions by aspiration cytology. Four findings appeared to be useful in this context: (1) malignant cell clusters with tubular structure, (2) cytoplasmic lumen formation in malignant cells, (3) fibroblast proliferation, and (4) fragments of elastoid stroma. When any combination of two or more of these key features was seen in a smear being diagnostic of malignancy, the positive predictive value regarding invasiveness was 96%. The accuracy of this prediction in terms of sensitivity and specificity was 48% and 96%, respectively. The clinical use of these observations is discussed.