That most cytotoxic agents act specifically against actively proliferating cells is well-recognized. In this study, we attempted to correlate pretreatment S-phase fractions (SPF) measured on DNA histograms with regression of the tumor mass after the administration of neoadjuvant chemotherapy. Tumor cells were obtained from 60 previously untreated, premenopausal patients with no metastases and with noninflammatory disease by fine needle sampling without aspiration. We could evaluate DNA ploidy in all patients and SPF in 50 or 83% of them. Tumor responsiveness was significantly related to SPF. The 12 patients who had SPF of 10% or more showed demonstrable regression; six had complete responses. None of the other parameters tested, i.e., DNA ploidy, histopathologic grade, or hormone receptor content, correlated with response. We believe this information may prove valuable for clinicians as they make their decisions regarding patient therapy.