The suppression of local estrogens levels is of key importance in the treatment of ER-positive breast cancer. Essentially all endocrine strategies act by either suppressing estrogen formation or competitively inhibiting receptor-binding in tumor cells. Nevertheless, little is still known about the local expression of aromatase and sulfotransferase which are the key modulators of intra-tumoral estrogen levels. We have performed immunohistochemostry to investigate the expression of aromatase and sulfotransferase in 42 samples obtained directly from malignant breast tumors, and compared it to biopsies obtained from uninvolved tissue in the vicinity of the invasion front, and to distant breast tissue. We found that aromatase was equally detectable in both tumor epithelial and stroma, but was mostly epithelial in non-malignant tissues (P=0.00008, Fisher's exact test). Also, aromatase protein expression was significantly more common in tumoral stroma when compared with peritumoral and distant breast stroma (P=0.00005, and P<0.00001 respectively). With the notable exception of cystosarcoma phylloides, sulfotransferase protein was detectable only in epithelial tissues, regardless of the location within the diseased breast. However, epithelial sulfotransferase was correlated with epithelial aromatase (r=0.35461, P=0.0009, Spearman's rho test) and with the epithelial ER status (r=0.29313, P=0.005). We have demonstrated a differential aromatase and sulfotransferase protein expression pattern that is dependent on the spatial relation to a malignant breast tumor. Our results indicate a net increase in intratumoral active estrogen levels through increased stromal aromatization, while physiological local inactivation by sulfotransferase activity remains essentially unchanged.