Combinations of dietary factors were studied in relation to breast-cancer Occurrence among I33 breast cancer cases and 289 population controls in The Netherlands. Dietary factors were classified according to their possible mechanism of action, Le., relating either to the intestinal microflora (total fat, fiber, fermented milk products) or to the anti-oxidant hypothesis (beta-carotene, selenium and polyunsaturated fatty acids). From 6 interactions evaluated, the combination of high fiber intake and high intake of fermented milk products was the only one suggesting synergistic protection (ageand-fat-adjusted OR for interaction = 0.48, 95% confidence interval (CI) = 0.2 I -I. 13). In order to estimate the extent to which the above dietary factors together might be related to breast cancer, subjects with a supposedly favorable dietary pattern (low fat intake, high fiber intake, high intake of fermented milk products; high intake of beta-carotene and selenium, low intake of polyunsaturated fatty acids) were compared with subjects with an unfavorable dietary pattern. This resulted in an age-adjusted odds ratio of 0.40 (95% CI = 0. I 4 -I. IS), which was largely attributable to the combination of low intake of fat and high intake of fermented milk products and fiber (age-adjusted OR = 0.33, 95% CI = 0. I 5 -0.73). The other factors did not appreciably affect the odds ratio. These analyses show in a quantitative way that a dietary pattern which combines low intake of fat and high intake of fiber and fermented milk products might provide substantial protection against breast cancer.