Flow-cytometric DNA analyses of single cell nuclei were performed on nuclear suspensions prepared from biopsies of primary breast carcinomas in 638 patients. Propidium-iodide-stained cell nuclei were analysed in an Ortho 50-H Cytofluorograph. The patients were staged by the TMN classification. Sixty percent of all the patients had aneuploid primary breast carcinomas; of size T1, T2, T3, and T4 tumors, 51%, 63%, 67%, and 75% were aneuploid, respectively. The proportions of aneuploid tumors in each of the tumor stages SI, SII, SIII, and SIV were 47%, 62%, 67%, and 69%, respectively. This trend to increasing aneuploidy proportions with more advanced disease was significant in contrast to the degree of aneuploidy found in relation to axillary nodal tumor involvement. Multiple aneuploid cell populations were found in 109 (17%) tumors. With a mean follow-up time of 16 months, 92 patients have relapsed out of 540 completely staged patients with unilateral breast cancer with no distant metastases at the time of initial treatment. When the influence of various treatments and tumor stage are not considered, the recurrence rate was twice as high among patients with aneuploid primary tumors than among patients with euploid tumors. The differences in relapsing rates among patients with euploid and aneuploid primary tumors decreased with more advanced disease. Out of 170 patients with T1 tumors, 17 relapsed and 16 of these were aneuploid. No such difference in relapse rate in relation to ploidy was, however, found in patients with more advanced primary disease.