A displacement assay with tamoxifen, based on the relative binding affinity of tamoxifen and estradiol for the estrogen receptor (ER), was proposed in 1990 as prognostic indicator for breast-cancer patients. Validation of its predictive results in relation to the outcome of 73 patients with ER ؉ tumors is analyzed. ER, progesterone receptor (PgR) determinations and other conventional prognostic factors in relation to the displacement assay, were considered. Displacement assay results allowed ER ؉ tumors to be grouped as displaceable (D) or weakly displaceable (WD), with the implication that D tumors should respond better to tamoxifen (Tam) administration. Survival and disease-free interval curves showed highly significant differences between patients with ER ؉ D and ER ؉ WD tumors. For survival, including all tumor stages, 73.9% of patients were alive at 9 years after surgery in the group with D tumors and 37.0% in the group with WD tumors (p F 0.005); relative contribution of the different stages is analyzed. Addition of axillary-node number increased the prognostic significance of displacement categories for survival and disease-free interval. PgR determination as another ER functional expression failed to show significant differences for survival and disease-free interval between ER ؉ PgR ؉ and ER ؉ PgR ؊ tumors. Thus, results from the displacement assay and from PgR determinations reflect 2 independent ER functional expressions. Displacement assay data appear as reliable prognostic indicators of breast-cancer outcome, and contribute to more appropriate treatment decisions in this pathology.