A review of over 2000 patients who had estrogen receptors (ER) assayed in the primary breast carcinoma identified 48 cases in whom a subsequent second primary breast carcinoma or concurrent or recurrent secondary tumour had been tested for ER status. The relationship between the ER in the two specimens was as follows: Of 14 concurrent primary and secondary breast carcinomas the ER concentration was the same in 11 cases; in 1 case it was significantly higher in the primary tumour, in 2 others the reverse was observed. There was no major discordance in ER status. In 14 sequential carcinomas (after an average disease free time of 21 months), 12 pairs had identical ER status. There was major discordance of ER status in 2 cases where the secondary tumours contained ER while the primary carcinoma did not. The ER concentrations in the primary and the secondary carcinomas were comparable in 8 cases, while 3 and 5 cases had significantly higher or lower concentrations respectively in the sequential secondary tumour. In 20 cases where breast cancer developed in the contralateral breast (after an average disease-free interval of 27.7 months), essential concordance of ER status was observed in 15 of 20 sequential carcinomas. In 5 patients the first carcinoma was ER- and the second ER+; in one additional patient the first carcinoma was ER +/- and the second ER-. The ER concentrations differed significantly in 14 of the 20 bilateral carcinomas. The literature on estrogen receptor variation in breast carcinoma was reviewed.