The estrogen receptor (ER) level in carcinoma of the breast is a useful predictor of response to hormonal therapy. Metastatic disease may not have the same level of ER as the primary. In a series of 37 patients who had simultaneous ER determinations in both primary neoplasm and regional nodal metastasis there was 81% agreement. The true ER character of the tumor would have been missed in seven patients if only the primary tumor had been sampled. The possible reasons for this discrepancy are discussed. We feel that the nodal metastatic deposit may reflect the true nature of the ER status because it represents a purer concentration of tumor cells as well as representing the aggressive element of the tumor.