Abstract To determine if the flow cytometric measurement of the content of the tumor DNA could provide prognostic information in T 1N 0M 0 and T 2N 0M 0 carcinoma of the breast, we isolated nuclei from paraffin-embedded tumor specimens from 128 consecutive patients who underwent modified radical mastectomy and were found to have a T 1 or T 2N 0M 0 carcinoma of the breast. The content of DNA of the nuclei was determined by flow cytometry. Although the average tumor size was not significantly different, 17 of 56 patients with aneuploid tumors died of cancer compared with 2 of 72 with euploid tumors. The 10-year overall actuarial survival rate of the euploid and aneuploid groups was 72% and 67%, respectively (p < 0.02). A hazards model of the data shows that the DNA content of the tumor is the most significant indicator of prognosis, with a 2.25-fold increased risk of death for patients with aneuploid tumors. These data show that the tumor DNA content can be used to identify a group of patients with T 1 or T 2N 0M 0 carcinoma of the breast with the same prognosis as a group of patients without carcinoma of the breast that would be unlikely to benefit from adjuvant therapy.