The aromatase complex has a key role in regulating oestrogen formation in normal and malignant breast tissues. Using dexamethasone-treated fibroblasts, derived from breast tumours, breast tumour cytosol and breast tumour-derived conditioned medium (CM) markedly stimulate aromatase activity. The cytokine, interleukin-6 (IL-6) has been identified as a factor present in CM which is capable of stimulating aromatase activity. To examine whether IL-6 may have a role in vivo in regulating breast tissue aromatase activity, IL-6 production and aromatase activity in breast tumour and adipose tissue from breast quadrants were examined. In 5/6 breasts examined so far, aromatase activity was highest in adipose tissue in the breast quadrant containing the tumour or on which the tumour impinged. There was a significant correlation (P < 0.05, Kendall's rank correlation) between IL-6 production and aromatase activity in these breast tissues. It is concluded that IL-6 may have an important role in regulating aromatase activity in breast tissues.