A series of 327 breast cancers was analyzed for DNA ploidy by flow cytometry from paraffin-embedded tissue, and the resulting DNA histograms were classified independently by 6 researchers in the field as DNA diploid (Di), aneuploid (An), tetraploid (Te), multiploid (Mu), or technically uninterpretable. The frequency of diploid, aneuploid, tetraploid and multiploid cancers varied from 28 to 41%, 33 to 49%, 8 to 21% and 2 to 6%. According to the scale Di-An-Te-Mu, DNA ploidy was not significantly associated with breast-cancer mortality by 2 classifiers, but if DNA euploid cancers (Di+Te) were tested against non-euploid, or diploid cancers against non-diploid, all classifiers found DNA euploid and diploid cancers to have better prognosis. Mortality associated with diploid or tetraploid cancers decreased with improving histogram quality and increasing uniformity of classification, whereas that associated with aneuploid cancers remained unaltered. Among the cases where all classifiers agreed on ploidy, tetraploid, diploid and aneuploid cancers were associated with 100%, 88% and 68% 5-year survival rates. In this sub-set the S-phase fraction and possibly DNA ploidy were independent prognostic factors, together with histological grade, axillary node status, and primary tumor size.