Of 163 human breast cancers examined, 68% contained detectable aneuploid populations, whilst 32% had apparently normal DNA distributions. A slightly higher incidence of aneuploidy was observed in pre-menopausal patients (83%) than in post-menopausal patients (66%). Also, pre-menopausal patients had slightly higher proportions of S-phase or cycling (S + G2 + M) cells. Estradiol receptor negative (ER –) tumours from post-menopausal patients were found to have the lowest incidence of aneuploidy (59%) and ER– tumours from pre-menopausal patients the highest (91%). There were no significant differences in the proportions of cycling nuclei when receptor status and menopausal status were considered. A weak relationship is shown to exist between flow cytometric data and two common prognostic indicators of breast cancer, namely receptor status and menopausal sta