Sixty-two breast carcinoma patients underwent mastectomy and were analyzed for correlations between estrogen receptor (ER) status of the tumor and the histological findings of tumor, age, nodal status, and tumor size. An immunocytochemical assay for the detection of ER in breast carcinomas was used to evaluate the staining intensity index (SII) of the ER level of the tumors, which was calculated from the staining intensity of cancer cells and the percentage of ER-positive cancer cells. The histological differentiation of carcinomas showed a strong correlation with the SII. A statistically significant (F = 10.580, P less than 0.0005) negative correlation was found between the grade of malignancy and the SII. Patients under 50 years of age tended to have a lower SII, whereas patients over 49 years of age tended to have a higher SII. The tumor size, and nodal status showed no correlations with the SII in any of the patients. The histological differentiation of the tumor cells showed a strong correlation with the heterogeneity of the distribution of ER-positive cells in breast cancers.