One hundred and five cases of pure ductal carcinoma in situ (DCIS) seen in the Guy's Hospital breast unit between 1975 and 1991 were reviewed. The presence and extent of necrosis and the degree of cytonuclear differentiation were assessed and 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 evaluated using immunohistochemical methods. A strong correlation was seen between the presence and extent of necrosis and the degree of cytonuclear differentiation and between both these morphological criteria and the biological markers as well as between the individual markers. The presence of extensive necrosis was associated with lack of cytonuclear differentiation and both were associated with a high proliferation rate, the presence of cerbB2 and p53 protein, and the absence of progesterone receptors. In cases with little or no necrosis, there was good nuclear differentiation and a strong correlation with the presence of progesterone receptor, absence of cerbB2 and p53 protein, and a low rate of proliferation.