The expression of aromatase was evaluated in 38 breast carcinomas by an immunohistochemical method (ABC) using an specific polyclonal antibody against human placental aromatase. Fifteen tumors (40%) showed significant immunoreactivity, as defined by cytoplasmic positivity of moderate intensity present in at least 15% of the cells. The results were correlated with the estrogen and progesterone hormone receptor status and several clinicopathologic parameters such as age, tumor size, lymph node status, and stage of the disease. There was a significant, but inverse, correlation between the aromatase activity and the estrogen receptor status (P = 0.04), indicating the likelihood of negative estrogen if substantial aromatase activity was present. No statistically significant correlation was found between the presence of intratumoral aromatase and the rest of the parameters studied (P greater than 0.7). Nor was there a correlation between the aromatase content of the tumors and the menopausal status. The degree of intratumoral heterogeneity of the aromatase content was minimal in six cases where multiple samples from each tumor were analyzed. This is the first study reporting the detection of aromatase in archival material from breast carcinomas using immunohistochemical techniques. The lack of biologic significance of its presence in breast cancer reported here and by others using biochemical assays should be validated in larger series with longer follow-up. The method described can be readily used for that objective.