The prospective prognostic significance of flow cytometry derived DNA-ploidy status, the level of the S-phase fraction (SPF), estrogen receptor (ER) content, and combinations of these factors, was evaluated with respect to overall survival (OS) in a series of 516 breast cancer patients who were without signs of residual or distant disease after primary completed treatment. The median duration of survival follow-up time was ten years (range, 95-148 months) for surviving patients. Of the single factors, ER was the only significant predictor among node-negative patients; the ten-year OS rate was 71% in cases with ER-rich tumors vs. 62% for ER-poor tumors (p = 0.03). Where tumors were both non-diploid and ER-poor, the ten-year OS rate was 58%, as compared to 75% for the remaining node-negative patients (p = 0.003), who constituted a low-risk group whose survival was comparable with that in the age-matched normal population. Among patients with 1-3 positive nodes, the ten-year OS rate was 65% in patients whose tumors had an SPF < 7.3% vs. 50% if the SPF was > or = 7.3% (p = 0.01), and 58% in cases with ER-rich tumors vs. 45% where the tumors were ER-poor (p = 0.02). In a multivariate analysis, apart from age and menopausal status the combination of ploidy status and ER content was the significant (p = 0.002) predictor of OS in node-negative patients. Thus, combining ploidy and ER status, both of which are variables easily determined, enabled the selection of a subgroup of patients at high risk of relapse and reduced survival whose prognosis should be improved by effective adjuvant systemic treatment, whereas the remaining low risk N0 patients can not be expected to derive any survival benefit from adjuvant therapy since their predicted survival is already on a par with that of the general population.