A series of 71 patients undergoing surgery for primary breast carcinoma was prospectively studied in order to evaluate the relative weight for four biologic factors (intermediate filament vimentin expression, proliferating cell nuclear antigen [PCNA], flow cytometric DNA ploidy and S-phase fraction) and of several clinicopathologic and biologic features in predicting clinical outcome (disease-free interval). In univariate statistical analysis, positivity of axillary nodes, high number of mitoses, high nuclear grade, high histologic grade, positivity of vimentin, high flow cytometric S-phase fraction (FCM-S) value, high PCNA and high silver-stained nuclear organizer regions scores were significantly related to risk of relapse. In multivariate analysis (Cox's logistic regression) only histologic grade (3) and high FCM-S values (> 10.7) were independently related to risk of relapse, with hazard ratios of 9.84 and 7.98, respectively. The results of our preliminary, prospective study suggest that FCM-S, in addition to morphologic criteria (histologic grade), may be an important biologic indicator in determining breast cancer patients' prognosis.