The DNA content of paraffin embedded primary tumour tissue has been measured by flow cytometry in 354 patients with operable breast cancer. Tumour ploidy significantly correlated with tumour size, histological grade, and with menopausal status. No significant correlation with oestrogen receptor status or lymph node involvement was found. Patients with diploid cancers had a significantly improved short term survival and disease-free interval (DFI) compared with patients having aneuploid tumours. However, no difference in survival or DFI was shown after longer term follow-up (median 84 months). Multivariate analysis showed no independently significant prognostic value for tumour ploidy. No patient in this study received adjuvant therapy.