Rev Assoc Med Bras 2009; 2010; 56(6): 651-5 *Correspondence Rua Dr. Cesário Mota Jr, no 112 São Paulo SP Brazil CEP: 01221-020 AbstrACt Objective. To evaluate aromatase enzyme expression in invasive ductal carcinoma (IDC) and ductal carcinoma in situ (DCIS) present in the same breast in adjacent epithelium and stroma. Methods. Forty-five surgical samples were collected from mastectomies and quadrantectomies from patients with simultaneous stage I and II IDC and DCIS. Aromatase enzyme expression analysis used anti-aromatase polyclonal antibodies. The samples were classified by number and intensity of stained cells. Results. Aromatase expression was positive in 32 (71%) IDC cases and negative in 13 (29%). The same results were obtained in the DCIS, showing a perfect positive correlation. In normal epithelium, aromatase expression was positive in 19 (42.2%) and negative in 26 (57.8%) cases, a statistically significant positive correlation when compared to IDC and DCIS (p < 0.01). Analysis of normal stroma revealed only 7 (15.5%) of the 45 cases of positive expression, showing no correlation with any variables analyzed for aromatase expression. As for tumor stroma, aromatase expression was positive in 36 (80%) and negative in 9 (20%) of cases, a statistically significant correlation with IDC (p < 0.01) and DCIS (p < 0.01) expression. No statistically significant differences were found by comparing aromatase expression results in IDC, DCIS, normal epithelium and tumor stroma with nuclear grade, histological grade, tumor size and age. cOnclusiOn. Results showed high levels of correlation between aromatase expression in IDC, DCIS, normal epithelium and tumor stroma, suggesting the enzyme has a possible autocrine and paracrine mechanism in breast cancer.