Cytokines, such as IL-6 and tumor necrosis factor (TNF)-alpha, have an important role in regulating estrogen synthesis in peripheral tissues, including normal and malignant breast tissues. The activities of the aromatase, estradiol 17beta-hydroxysteroid dehydrogenase and estrone sulfatase are all increased by IL-6 and TNF-alpha. Prostaglandin E2 may also be an important regulator of aromatase activity in breast tumors. Macrophages and lymphocytes, which invade many breast tumors, are thought to be an important source of factors that can stimulate estrogen synthesis in malignant breast tissues. The co-ordinated stimulation of the activities of the enzymes that are involved in estrogen synthesis offers an explanation for the high concentrations of estrogens that are present in breast tumors.