Women with palpable breast cysts may have a higher risk of developing breast cancer. High concentrations of androgens and oestrogens have been found in breast cyst fluid, some of which may be implicated in mammary carcinogenesis. The aim of this study was to investigate whether breast cysts were capable of metabolizing radiolabeled dehydroepiandrosterone sulphate and, if so, to identify the metabolites formed. Breast cysts were found to possess the enzyme systems required for converting dehydroepiandrosterone sulphate to dehydroepiandrosterone, androstenedione, androstenediol, and testosterone. Steroid metabolism by breast cysts may play a role in the development of breast cancer.