Aspiration cytology was performed on 2772 breast masses, all of which subsequently had open biopsies. Of the 1745 histologically malignant tumors, 1539 (88%) had a concordant cytologic diagnosis: 54 (0.3%) were diagnosed as probably malignant; 63 (3.6%) were false negatives; and 80 (0.5%) had inadequate smears for diagnosis. Of the histologically benign lesions, 916 (89%) had a concordant cytology: only 3 (0.3%) were false positives; cancer was suggested in 42 (0.4%); and the smears were inadequate in 66 cases (6.4%). Very small or very large sized cancer and a high degree of differentation were major causes of false negative aspirates. Recurrent tumors in those breasts exclusively treated by radiotherapy were documented by cytology in 56 of 69 cases (78%). Aspiration cytology is highly reliable when the diagnosis of cancer is made, but should be ignored if no malignant cells are observed.