Morphologic and cytologic features and preliminary survival data were correlated with the presence or absence of estrogen receptor protein in tissues from 123 primary and 26 metastatic carcinomas of the breast. The estrogen receptor positive tumors were somewhat smaller (2.8 cm in diameter) than the estrogen receptor negative tumors (3.2 cm), but the rate of axillary lymph node metastasis was similar. Fifty-eight percent of the infiltrating ductal carcinomas, 64% of 11 infiltrating lobular carcinomas and all 8 less common cell types including 4 mucinous, 2 papillary, 1 adenoid cystic and 1 tubular carcinoma were estrogen receptor positive. Twenty-one of 33 aspiration smears performed were cytologically positive, the remaining 12 suspected of being carcinoma. Eleven of the 14 estrogen receptor negative tumors that were aspirated were positive, possibly due to a lesser differentiation. Sixty percent of the patients who 9-40 mo. after operation are living without known recurrent disease had estrogen receptor positive tumors, but only 28% of the 18 patients who have died of carcinoma of the breast or are living with known recurrent disease were in this category. In spite of a similar rate of axillary lymph node metastasis and without regard to the modality of treatment, estrogen receptor negative tumors tend to have a less favorable course.