In a prospective study using mammographically guided fine needle aspirates in 100 nonpalpable breast lesions, the author's group assessed the reliability of a cytological grading system to define the cytological features of proliferative and nonproliferative breast disease and to differentiate between benign, premalignant and malignant breast lesions. We developed a cytological grading system evaluating the aspirates for the cellular arrangement, the degrees of cellular pleomorphism and anisonucleosis, presence of myoepithelial cells and nucleoli and the status of the chromatin pattern. This grading system, now recognized as the Masood Cytology Index, is commonly used as a surrogate end point biomarker in chemoprevention trials.