Nuclear DNA content measured by a cytofluorometric method was investigated to evaluate its clinical significance in 73 cases with breast cancer which had extended radical mastectomy. Many cases of D type (diploid type) which had a prominent peak at the 2c region (control DNA content of lymphocytes) were safe indications for modified radical mastectomy. Bone recurrence was significantly more frequent in D type than in N type (non-diploid type) which did not have a prominent peak at the 2c region. Because D type had high positive rate of hormone receptors, endocrine therapy might be effective for many cases. Local or lung recurrence was more common in cases with N type which had a significant low cumulative survival rate compared with D type and especially N type 4c-3 of which over 4c cells more than 30% had the poorest prognosis. Therefore, multidisciplinary treatment composed of extended operation, radiation and chemotherapy was necessary to improve the prognosis of breast cancer patients with N type 4c-3. It was concluded that the measurement of nuclear DNA content in breast cancer might be useful for decision of therapy suitable for each case based on malignancy grading.