The aim of the study was to correlate the DNA Index (DI) and S-phase fraction (SPF) values determined by multiparametric flow cytometry in breast cancer (mainly T1 and T2 stages) with several clinico-pathologic variables and other biological parameters. For this purpose, a total of 136 breast cancers were submitted to flow cytometry and to several types of immunohistochemical analyses. Among clinico-pathologic data we considered pT, pN and grade and among immunohistochemical markers, hormonal receptors, P53, c-erbB-2 and MIB-1. We found that DNA aneuploidy was strongly correlated with high tumoral grade, absence of hormonal receptors, high proliferation, as shown by high MIB-1 (> or = 36%) and SPF values (> or = 13.3%), and P53 positivity. Grouping the tumours according to their DI values, we observed a relative significantly higher correlation of the near-triploid range carcinomas (DI 1.40-1.60) with immunohistochemical expression of P53 (p = 0.0001). Near-triploid DI was also associated with a high proliferative activity, expressed both by SPF (p = 0.0001) and MIB-1 reactivity (p = 0.0001), high tumoral grade (p = 0.0001) and presence of axillary metastases (p = 0.03). These data suggest that DNA near-triploid tumours in breast cancer may have a more aggressive behaviour in comparison with other DI classes.