Data from a multicenter case-control study on breast cancer conducted in Italy have been used to analyze the relationship of olive oil and other dietary fats to breast cancer risk. Cases were 2,564 women hospitalized with histologically confirmed, incident breast cancer. Controls were 2,588 women admitted to the same network of hospitals for acute, non-neoplastic, non-hormone related, non-digestive tract disorders. Cases and controls were interviewed between 1991 and 1994 using a validated food-frequency questionnaire. The data were modelled through multiple logistic regression controlling for demographic and reproductive breast-cancer risk factors, energy intake and, mutually, for types of dietary fat. For olive oil, compared with the lowest quintile, the odds ratios (OR) were 1.05, 0.99, 0.93, and 0.87 for increasing quintiles of intake; in a model postulating linear logit increase, the OR per unit (30 g) was 0.89 (95 percent confidence interval [CI] = 0.81-0.99, P = 0.03). Among other oils or fats considered, the OR for the highest level of intake was 0.72 (CI = 0.6-0.9) for a group of specific seed oils (including safflower, maize, peanut, and soya) compared with nonusers. The ORs for the highest cf lowest level of intake were 0.80 for mixed or unspecified seed oils, 0.95 for butter, and 0.96 for margarine. The study, based on a large dataset from various Italian regions, shows an inverse relationship of breast cancer risk with intake of olive oil and other vegetable oils, but not with butter or margarine.