One-hundred-and-forty fresh frozen breast tissue samples from Chinese patients collected from May 1991 to October 1992 were evaluated for DNA content by flow cytometric analysis. None of the 12 benign tumors displayed aneuploidy. One-hundred-and-twenty-eight of the 140 specimens were histologically proven to be malignant. Ploidy was compared to the clinical characteristics which included age, tumor size, lymph node involvement, menopausal status, steroid receptor status, histological grading and pathological staging. After univariate and multivariate analyses, aneuploidy was more frequently identified in progesterone receptor negative and high grade tumors. This study suggests that an aneuploid DNA content in a breast cancer specimen may indicate more aggressive cancer.