The cytochrome P-450 enzyme complex aromatase is the rate-limiting step in the production of oestrogens. It catalyses the conversion of androgens to oestrogens. In the treatment of hormone-dependent breast cancer in postmenopausal women, aromatase is the target for treatment with aromatase inhibitors. Recently registered aromatase inhibitors like anastrozole, letrozole and exemestane have proven to be effective therapy for advanced breast cancer in postmenopausal patients failing to respond to treatment with tamoxifen. Intratumoural aromatase activity has predictive value for response to treatment with aromatase inhibitors. Attempts are being made to find an immunohistochemical technique to determine aromatase in tumour tissue, which may serve as a predictive factor. In situ oestrogen synthesis through local aromatase activity in the tumour and adjacent tissue is probably a very important growth-stimulating system in hormone-dependent breast cancer. This synthesis can be blocked with aromatase inhibitors. The regulation of aromatase activity and the cell types that contribute to this process are the subject of extensive research. There seems to be a complex interaction between malignant cells and adjacent cells in which factors such as IL-6 and its soluble receptor, TNF-alpha and prostaglandin E2 play an important role in stimulating aromatase activity.