In a population of 110 primary breast cancers with medullary features, registered in the Danish Breast Cancer Cooperative Group (DBCG) from 1977-82, we have determined ploidy and S-phase fraction (SF) by flow cytometry (FCM) on paraffin embedded tumour tissue. The distribution of DNA ploidy is not different from the distribution described for breast cancers in general. No difference is found between the subgroups of medullary and non-medullary cancer when using a new simplified histopathological definition of medullary carcinoma of the breast, recently proposed by us. When using the definition proposed by Ridolfi et al. in 1977, we find significantly more tumours with aneuploidy and high SF in the groups of typical medullary carcinoma (TMC) and atypical medullary carcinoma (AMC) than in the small group of non-medullary carcinoma (NMC), which seems a paradox, as patients with NMC have the worst prognosis. However, the number of patients in the NMC group is very small, and the percentage of aneuploid tumours is very low. In 84 protocolled patients we found no statistically prognostic importance of ploidy or SF, either in the whole group assessed or when stratifying for the histopathological subgroups. However, a prognostic influence of SF can be traced for the non-medullary cancers, according to the new definition, but not for the medullary cancers of the breast. The result emphasizes the impression of MC as being biologically different from other histological types of breast cancer.