Results of international correlation and migrant studies suggest that dietary fat promotes carcinogenesis in hormone-sensitive sites, but this is disputed. In the present study, we used a Noble rat model of sex hormone-induced cancers to examine the effect of a high-fat diet on the incidence and latency of prostate and mammary cancer in male (n 139) and female (n 72) animals respectively. We also measured alpha-tocopherol levels in female breast tissue to determine whether a high intake of polyunsaturated fatty acids depletes antioxidant defence in target tissues, providing a possible potentiating mechanism for carcinogenesis. Results showed a very high incidence of hormone-induced adenocarcinomas of prostate and mammary gland, irrespective of diet. There was no difference in the pattern of carcinogenesis in different prostatic locations, weight of the prostate, or weight gain between male rats on the high-fat diet compared with the control (standard, low-fat) diet. In female rats, the incidence of mammary cancer and the body-weight gain were the same in both dietary groups, and breast alpha-tocopherol was also unaffected by dietary fat intake. Our present results are supportive of recent cohort studies that reported no significant association between intake of fat and the development of human prostate and breast cancer, and do not support a role for dietary fat in promoting sex hormone-induced prostate and mammary carcinogenesis.