Quantitative analyses (QAs) of prostatic intraepithelial neoplasia (PIN) helped to objectively define some traditional features of the lesion because, first, changes in value are represented by numbers and not by subjective evaluation of morphologic clues. QAs have also helped to identify subtle abnormalities. For example, the degree of nucleolar margination is a new diagnostic feature which can be easily evaluated as the proportion of nucleoli touching the nuclear membrane. Thirdly, QAs have provided useful insights into understanding some morphologic changes. PIN, in fact, appears to be characterized by complex changes which involve the secretory cells as well as the basal layer and which affect the surrounding stroma. In the epithelial lining, two types of simultaneous changes take place, the first in the nucleus (expression of abnormal proliferative and/or renewal activity) and the second in the cytoplasm (expression of the disorder in cell differentiation), pointing out that only PIN samples of high grade can be considered as having acquired the characteristics of a neoplastic lesion.