The DNA ploidy of breast cancer tissue from paraffin blocks was measured by flow cytometry in 117 patients whose disease had been detected and treated with surgery between 1974 and 1976. Patients with aneuploid tumours had positive axillary nodes and distant metastases more often than those with diploid tumours. Aneuploid tumours were more common in postmenopausal than premenopausal women. The S-phase fraction (SPF) was significantly higher in aneuploid than in diploid tumours and positive axillary lymph nodes were found in 26 per cent of the patients who had a tumour with a SPF below the median (4.8 per cent) and in 48 per cent of those with tumours with SPF values above the median. At the primary clinical investigation 2 per cent of the patients with diploid tumours and 6 per cent of those with aneuploid tumours had distant metastases. During the follow-up, the proportion of patients with distant metastases increased to 42 and 72 per cent, respectively. With a follow-up of 11.5 years, the DNA aneuploidy of the tumour showed a significant association with decreased survival. Thirty-three per cent of patients with diploid and 65 per cent of patients with aneuploid tumours had died from breast cancer during the follow-up (P less than 0.001). All patients with hypertetraploid or multiploid tumours died from breast cancer. High SPF values were associated more closely with distant metastases or death during the follow-up than low SPF values. Our results suggest that DNA ploidy measured by flow cytometry from paraffin embedded tissue blocks of human breast cancer can be used to predict the aggressiveness of the tumour and the survival of the patients.