One hundred five cases of pure ductal carcinoma in situ (DCIS) seen in the Guys Hospital breast unit between 1975 and 1991 were reviewed and reclassified using a modified histologic classification based on cytological features as well as histological architecture. The expression of p53 protein, cerbB2 protein, progesterone receptor, and a proliferation antigen KiS1, all factors reported to be of prognostic significance in invasive ductal carcinoma, was also evaluated using immunohistochemical methods. The mode of presentation of these cases was noted, and its relationship to biological markers and histologic type was also assessed. Good interobserver agreement was achieved by two independent observers using the modified histologic classification. Strong correlation was seen between histologic pattern and biological markers as well as between the individual markers. Poorly differentiated DCIS was associated with a high proliferation rate, the presence of cerbB2 and p53 protein and the absence of progesterone receptors. Well-differentiated DCIS showed the reverse, and the intermediate group showed an intermediate pattern. Paget's disease of the nipple was only seen in association with poorly differentiated DCIS, but no other significant association was noted between mode of presentation and DCIS type.