The effect of dietary omega-3 fatty acids (fish oil) on azoxymethanol-induced focal areas of dysplasia and colon tumor incidence. MaxEPA (MA), a fish oil high in omega-3 fatty acids, was combined with various levels of corn oil (CO), rich in omega-6 fatty acids, and fed to female CF1 mice. The three fish oil blends with CO and the two CO levels of the diets studied were as follows: 16.0% CO + 4.4% MA (Diet 1); 10.2% CO + 10.2% MA (Diet 2); 4.4% CO + 16.0% MA (Diet 3); 20.4% CO (Diet 4); and 4.4% CO (Diet 5). The diets were provided 2 weeks before weekly subcutaneous injection of saline or azoxymethanol (AOM). Studies of epithelial cell proliferation and the incidence of focal areas of dysplasia (FAD) involved six weekly AOM injections. One week after the last AOM injection and 1 hour before killing, mice were injected with tritiated thymidine (3HTdR). No differences in any proliferative parameters were found among the five groups of saline-treated mice. Among the AOM-treated animals, those fed Diet 3 showed significantly fewer cells per crypt and significantly fewer labeled cells/gland than CO Diets 4 and 5. Additionally, the distribution of S-phase cells in crypts of AOM-treated mice fed Diet 3 most closely resembled that of the saline controls. The greatest alteration in the distribution of proliferative cells was observed in the high-CO diet (Diet 4) and the lowest MA level (Diet 1). Mice fed Diets 2 and 3 had significantly fewer FAD/500 microns of distal colonic serial sections than those fed the high CO diet (Diet 4). Mice involved in chronic tumor incidence studies received only three weekly injections of the same dose of AOM. Regardless of diet, approximately 88% of all tumors arose in the distal colon. A significantly larger tumor-bearing population was observed in both the high-CO Diet 4 and the lowest MaxEPA (MA) diet (Diet 1) compared with the incidence in MA Diets 2 and 3 and the low-CO Diet 5. A diet with a ratio of omega-6 to omega-3 fatty acids of approximately 1.0 apparently prevented the development of an adenoma-type proliferative pattern thereby reducing FAD numbers and subsequent tumor incidence. 