From November 1981 to the end of August 1984, 456 patients with breast lesions underwent aspiration biopsy cytology (A.B.C.). This study includes 109 for whom the diagnosis was histologically confirmed at surgical biopsy. Seventy-five lesions were histologically proven to be malignant and 34 were benign. The accuracy of diagnoses with A.B.C. was; true positive 86.7 per cent (65/75) of the time, true negative 82.6 per cent (28/34) of the time, false negative 5.3 per cent (4/75) of the time and false positive 5.7 per cent (2/34) of the time. Unsatisfactory or inadequate aspirated tissue made A.B.C. diagnosis difficult in 5.5 per cent (6/109) of the cases. Three out of 7 with malignant tumors, who were wrongly diagnosed as benign by A.B.C., had tumors with a diameter of 1.0 cm or less. Two benign cases which were falsely diagnosed as malignant also had small tumors about 1.0 cm in diameter. Although A.B.C. is more reliable than other conventional supplementary diagnostic techniques (mammography, ultrasonography, etc.), it is important to carefully follow clinically questionable cases which appear negative, using A.B.C. © 1985 The Japan Surgical Society.