The nuclear DNA content of 351 breast carcinomas was determined by flow cytometry from paraffin-embedded tissue to assess the prognostic significance of DNA ploidy, the DNA index (DI) and the S-phase fraction (SPF). The minimum follow-up of the patients was 22 years, and they were all from a defined urban population. DNA ploidy correlated with histological type and grade, mitotic count and nuclear pleomorphism (P less than 0.0001), and also with axillary nodal status (P = 0.0005), tumour necrosis (P = 0.001), primary tumour size (P = 0.03), menopausal status (P = 0.004) and the presence of distant metastases at the time of the diagnosis (P = 0.04). Survival corrected for intercurrent deaths of the patients with a diploid tumour was better than that of the patients with a non-diploid tumour (P = 0.0001, 48% vs 28% at 25 years). SPF had prognostic significance in both axillary node positive and negative patients, but ploidy and DI only in the node negative group, and their significance was greater in post-menopausal than in premenopausal patients. Axillary nodal status, primary tumour size, histological grade and the type of tumour margin circumscription were the most important independent prognostic factors in Cox's multivariate analysis, and SPF had independent prognostic value, whereas ploidy and DI did not. It is concluded that DNA ploidy, DI and SPF have long-term prognostic significance in breast cancer.