In this prospective study, the independent prognostic value of DNA ploidy in combination with the major clinico-pathological characteristics (histological grade, nodal status, tumor size, estrogen and progesterone receptor status, number of tumors, multicentricity, lympho-vascular infiltration) was evaluated in a series of 399 breast-cancer patients. The mean follow-up time was 4.5 years. The DNA content was measured using image cytometry on fresh tumor samples. The overall survival of tetraploid and slowly proliferating diploid cases was significantly different compared with that of aneuploid and rapidly proliferating diploid cases (p = 0.0002). Thus, DNA ploidy combined with S-phase estimate (DNA histogram type) appeared to be good prognostic factors. In a multivariate survival analysis, DNA histogram type was not an independent prognostic factor unless the histological grade was excluded. This effect of DNA histogram type on survival was also observed among patients with grade-I or -I1 tumors and patients with small tumors. In conclusion, DNA histogram type was a valuable prognostic factor in univariate analysis, and provided independent complementary information for patients considered at low or intermediate risk by classical pathological findings.