One frequently used classification of flow cytometric DNA ploidy status (diploid versus nondiploid) was compared with a division into seven ploidy classes based on DNA index (DI) and number of cell populations (hypodiploid, diploid, near-hyperdiploid, hyperdiploid, tetraploid, hypertetraploid, and multiploid). The latter ploidy classification showed a better correlation with prognosis and other prognostic factors (i.e., lymph node involvement, estrogen and progesterone receptor status, and S-phase fraction). The improvement in correlation was mainly due to the identification of near-hyperdiploid cases (DI 1.00-1.14) which could be combined with the diploid cases to form a group with favourable prognosis. In contrast to cases with a small increase in DNA content (near-hyperdiploid), those with a small decrease of DNA content (hypodiploid) manifested a more aggressive disease. In multivariate analysis, S-phase fraction (SPF) was a more important prognostic factor than both the improved or the conventional ploidy classification.