S phase fraction (SPF) and DNA ploidy were related to disease outcome by a separate analysis of sex steroid receptor positive and negative tumours in a series of 232 patients with breast carcinoma followed-up for over 8 years in our clinic. SPF was significantly higher in receptor-negative tumours than in receptor-positive ones (p = 0.037). SPF predicted recurrence only in ER+ or PR+ patients (p = 0.02-0.003). Recurrence-free survival (RFS) was significantly related to SPF only in ER+ (p = 0.001) and PR+ (p less than 0.001) tumours. In survival analysis, ER+ (p = 0.002) and PR+ (p less than 0.001) patients were efficiently divided into prognostic groups by SPF, whereas in ER- and in PR- tumours SPF had only suggestive predictive value. In N- tumours, SPF predicted recurrence-free survival and disease-related survival in ER+ (p = 0.003) (p = 0.039) and in PR+ (p = 0.003) (p = 0.012) tumours, respectively, whereas in ER-, PR-, tumours, SPF had no predictive value. In pN+ tumours, SPF also predicted survival in ER+ (p = 0.03) and in PR+ (p = 0.024) tumours. Thus the prognosis of ER+ or PR+ tumours with an SPF less than 9% is favourable with a risk of death of about 20%, in contrast to that of about 70% in tumours with an SPF greater than 9% during the follow-up period. To conclude, the proliferation rate as measured by S phase fraction by FCM is a highly significant prognostic factor in breast cancer. The prognostic value of S phase fraction is confined to steroid receptor-positive tumours, whereas in receptor-negative tumours SPF has no predictive value. The results thus suggest that all women with steroid receptor-negative breast tumours and those receptor-positive tumours with an SPF higher than 9% should be subjected to postoperative adjuvant chemotherapy immediately.