Although there is a strong positive association between total fat consumption and colorectal and breast cancer risk, there is evidence that n-3 fatty acids, mainly found in fish oil, are protective. If this is so, we would expect to be able to detect an inverse correlation between fish consumption and colorectal cancer and breast cancer mortality. Mortality data for breast and colorectal cancer in 24 European countries were correlated with current fish and fish oil consumption and with consumption 10 and 23 years previously. In males there was an inverse correlation between colorectal cancer mortality and current intake of fish (P = 0.036), a weaker correlation with fish consumption 10 years earlier (P = 0.042) and none with consumption 23 years earlier (P = 0.12). The data were not statistically significant in females. There was no correlation at all between breast cancer mortality and fish or fish oil consumption at any time. It is concluded that fish consumption is associated with protection against the later promotional stages of colorectal carcinogenesis, but not with the early initiation stages. The results are discussed in terms of the role of prostaglandins in colorectal carcinogenesis.