There is preliminary evidence that polymorphism of apolipoprotein E (apoE, protein; APOE, gene), one of the key regulatory proteins in cholesterol metabolism, influences the pathobiology of carcinoma of the colon, prostate and breast and also primary tumours of the brain. This study was designed to determine whether APOE polymorphism is related to variation in the rate of tumour cell proliferation and clinical outcome in carcinoma of the breast. One hundred and eleven infiltrating ductal carcinomas, for which follow up data were available, were included in the study. Estrogen and progesterone receptor status (ER, PR) cell proliferation index (MIB- 1) and APOE genotypes were determined from paraffin-embedded tissue by standard methods. Positive correlations were found between grade and tumour size, grade and presence of metastasis, grade and MIB-1 expression, as well as between ER and PR. Survival correlated inversely with tumour size and the presence of positive lymph nodes. Both steroid receptors correlated inversely with MIB- 1 expression. PR positive status also correlated inversely with high histological grade and presence of lymph node metastases. APOE allele frequencies resembled those of the general population. No significant associations were found between possession of either APOE epsilon2 or epsilon4 alleles and the parameters investigated. Although there is evidence to suggest that APOE epsilon4 may predispose to the development of carcinoma of the breast our data do not support the hypothesis that APOE genotype influences the rate of tumour cell proliferation or the clinical course.