The prognostic significance of DNA flow cytometric examinations of 247 patients (mean age 57 years; 27-84) with operable breast cancer was analyzed. The findings were compared with biochemically determined steroid-hormone receptors and lymph node status. Forty-two percent of the tumors were diploid and 58% aneuploid; 32% of them had a high S-phase portion (greater than 5%; definition by the crit-level method). S-phase fractions were lower in diploid than in aneuploid tumors (2.5 +/- 2.2% versus 5.5 +/- 3.4%). Both with diploid tumors as well as tumors with a low S-phase fraction (less than 5%), recurrence-free survival was better than with aneuploid tumors (p = 0.02) and tumors with a high S-phase fraction (p = 0.004). A direct proportional relationship was found between the S-phase fraction and the mean recurrence-free interval. As regards overall survival, no significant differences have been detected so far, either for diploid and aneuploid tumors or tumors with low and high S-phase fractions. Using the Cox regression model, the prognostic significance of ploidy status and S-phase, as new factors, independent of age, lymph node and steroid-hormone receptor status, is shown. DNA flow cytometry is suitable for routine clinical use and should be used in particular as an independent prognostic factor for planning and stratification in adjuvant therapeutic studies.