Studies investigating the relation of diet to breast cancer have produced conflicting results. We hypothesized that dietary factors associated with breast cancer risk might differentially influence the HER-2 status of the cancers that develop, and investigated this hypothesis by analyzing the data of the ORDET prospective study. We analyzed 8,861 volunteer women residents of the Varese Province, Italy, for whom we had full data. By December 31, 2001, 238 cases had occurred in which HER-2 status was known. Four dietary patterns had been identified previously by factor analysis: salad vegetables (high consumption of raw vegetables and olive oil), prudent (cooked vegetables, poultry, fish), western (potatoes, meat, eggs, butter), and canteen (pasta, tomato sauce, wine). In our study, relative risks (RRs) of developing HER-2-positive and HER-2-negative breast cancers by tertiles of dietary pattern factor scores were assessed by multinomial logistic regression. The salad vegetables dietary pattern had a protective effect against HER-2-positive cancers (RR = 0.25, 95% CI 0.10-0.64, for the highest tertile; p(trend) = 0.001), much stronger than for HER-2-negative cancers (p(heterogeneity) = 0.039). This important finding that a salad vegetables dietary pattern protects mainly against a specific breast cancer subtype indicates that future studies on environmental/dietary risk factors should explicitly take account of the heterogeneity of breast cancer phenotypes.