The relationship between nuclear DNA content determined by cytofluorometry in the primary focus of breast cancer and the survival rate was analyzed to clarify the prognostic significance of nuclear DNA content in breast cancer. The relationships of the ploidy pattern and the frequency of polyploid cells (4c or above) with the survival rate were studied in patients who underwent extended radical mastectomy and were comparable in the clinical stage and other prognostic factors. The survival rate was significantly lower in those of the non-diploid type showing no prominent peak at 2c or those in whom the frequency of polyploid cells was more than 30% than in those of the diploid type with a prominent peak at 2c or those showing few polyploid cells, even when the disease stage (Stage II by TNM classification and stage III by Tnm classification), histological lymph node metastasis (n (+), n1 beta), and histological type (papillotubular carcinoma, scirrhous carcinoma) were identical. From these findings, nuclear DNA content is considered to be a parameter of the malignancy of breast cancer and to have clinical significance as an important prognostic factor.