We analysed ploidy and S-phase fraction (SPF) from 78 paraffin-embedded primary prostatic carcinomas by DNA flow cytometry. DNA aneuploidy and above median (4.2%) SPF were both associated with high tumour grade, large size of prostate and presence of distant metastases. Both aneuploidy and high SPF (greater than 4.2%) indicated short 10-year progression-free (P = 0.01 for ploidy and P = 0.0002 for SPF), overall (P = 0.004 and P less than 0.0001) as well as prostate cancer survival (P = 0.002 and P less than 0.0001). Ten-year overall survival rate was 45% in cases with SPF below 4.2% and 0% in those with higher values, whereas the corresponding prostate cancer-specific survival rates were 80% and 11%, respectively. None of the seven tumours with SPF above 12% showed an objective response to endocrine therapy, whereas 26/49 (52%) of those with lower SPF values responded (P = 0.01). DNA ploidy, tumour grade, T-stage or M-stage did not significantly correlate with endocrine responsiveness. SPF was also the best predictor of progression free survival in patients treated hormonally. In conclusion, detection of high SPF in prostate cancer may indicate lack of hormonal responsiveness and poor prognosis.