The association between individual foods and breast cancer has been inconsistent. Therefore, we examined the association between diet and risk of postmenopausal breast cancer by the alternative approach of dietary patterns. Dietary patterns were identified with factor analysis from food consumption data collected from a food frequency questionnaire in 1984. Relative risks were computed using proportional hazard models and adjusted for known risk factors for breast cancer. Between 1984 and 2000, we ascertained 3,026 incident cases of postmenopausal breast cancer. We identified 2 major dietary patterns. The prudent pattern is characterized by higher intake of fruits, vegetables, whole grains, low-fat dairy products, fish and poultry, while the Western pattern is characterized by higher intake of red and processed meats, refined grains, sweets and desserts and high-fat dairy products. Neither of the patterns was associated with overall risk of postmenopausal breast cancer. However, a positive association between the Western pattern score was observed among smokers at baseline (relative risk = 1.44, comparing top to bottom quintiles; 95% CI = 1.02-2.03; p for trend = 0.03). An inverse association was observed between the prudent pattern and estrogen receptor-negative cancer (relative risk = 0.62; 95% CI = 0.45-0.88; p for trend = 0.006). Among the major food groups, higher consumptions of fruits (relative risk for 1 serving/day increase = 0.88; 95% CI = 0.80-0.97; p = 0.009) and vegetables (relative risk = 0.94; 95% CI = 0.88-0.99; p = 0.03) were significantly associated with decreased risk for ER(-) breast cancer. In conclusion, we did not observe an overall association between the prudent or Western pattern and overall breast cancer risk. However, a Western-type diet may elevate risk of breast cancer among smokers, and a prudent diet may protect against estrogen receptive-negative tumors.