Dietary fat has a positive correlation with cancers of the breast and colon and probably other types of cancer. In experimental tumour systems, linoleic acid (LA) has tumour-promoting effects that are mediated, in part, through eicosanoid production. The feeding of fish oil has resulted in decreased concentrations of LA and arachidonic (AA) and increased concentrations of n-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Both n-3 fatty acids antagonize the production of eicosanoids from AA. Recent reports indicate that marine n-3 fatty acids inhibit the development and growth of several experimental tumours. The effect of n-3 fatty acids on 16-alpha-hydroxylation of oestradiol, a putative biomarker for risk of breast cancer, was studied in 25 women at enhanced risk. The control group of 25 high-risk women received vegetable oil. After 4 months, the level of oestradiol 16-alpha-hydroxylation was significantly reduced in women receiving fish oil, but no change was detected in the control group. Although the precise mechanism of action is unclear, these data suggest that use of marine n-3 fatty acids as chemopreventive agents in high-risk women requires further investigation.