A case-control study of the association of dietary fat and animal protein consumption with breast cancer was conducted between 1975 and 1980 on Oahu, Hawaii. Data from this study were used to explore the relation of selected foods and the interaction of nutrients and foods with other factors, such as body size, age at menopause, and ethnicity on the risk for breast cancer. The sample included 272 postmenopausal breast cancer cases and 296 neighborhood controls. Study participants included Japanese and Caucasian women, aged 45 to 74, who were residents of Oahu. There was a suggestion of a positive-dose response relation (P < 0.01) between sausage consumption and the odds ratio for breast cancer. Significant odds ratios for breast cancer were also found for higher intakes (above the 50th percentile) of diary items, sausage, and all meats combined. The dose-response relation for nutrients and foods tended to be stronger among women with a high Benn's index (kg/cm1.5182) compared to women with a low Benn's index. In general, subjects with high dietary intakes of fat and animal protein who were in the upper 50th percentile of body size were at the greatest risk for breast cancer. However, there was no evidence for an interaction between the dietary variables and body size, ethnic group, age at menarche, age at menopause, or age at first birth that would affect the odds ratio for breast cancer. These data suggest that women with both a high intake of foods rich in fat and animal protein and with a large body size are at increased risk for breast cancer.