We investigated the effects of the dietary n-3-to-n-6 polyunsaturated fatty acid (PUFA) ratio (n-3/n-6 ratio) on mammary carcinogenesis induced by 7,12-dimethylbenz[a]anthracene in rats by feeding them several types of dietary fat with a fixed PUFA-to-saturated fatty acid ratio. Dietary fat was fed to the rats as 10% of the total feed weight, starting two weeks before the initiation. An increase in the n-3/n-6 ratio did not suppress the incidence or reduce the latency of mammary tumor development. The number and weight of mammary tumors per tumor-bearing rat tended to be large in the group with an n-3/n-6 ratio of 7.84 compared with those in the other groups. As the n-3/n-6 ratios were elevated, the total number and weight of tumors increased gradually. The prostaglandin E2 (PGE2) concentration in mammary tumor tissue was markedly low in the group with an n-3/n-6 ratio of 1.03 compared with the group with an n-3/n-6 ratio of 0.01. In addition, PGE2 concentrations were almost constant when n-3/n-6 ratios were > 1.03. These results suggested that the increase in the n-3/n-6 ratio of dietary fat with the fixed PUFA-to-saturated fatty acid ratio cannot suppress the mammary carcinogenesis but can promote development of tumors, despite reduced PGE2 concentration in the tumor.