The prognostic impact of DNA content and S-phase fraction (SPF) of tumour cells was studied in 93 patients with primary breast cancer. Aneuploid DNA content and high SPF were clearly associated with poor differentiation state of tumours and absence of steroid, especially progesterone receptors. Aneuploidy and high SPF tended to become more common with increasing primary tumour size, with more extensive nodal involvement and with more advanced stage of the cancer. Patients with diploid tumours had a slightly longer disease-free interval and survival than those with aneuploid tumours, whereas below median SPF as compared to above median SPF was associated with significantly longer (P less than 0.01) relapse-free interval and survival in patients with stage II-III cancer. We conclude that the DNA analysis of tumour cells is a promising method for the estimation of prognosis in breast cancer patients.