Objective: Due to the growing interest in the role of dietary patterns (DPs) on chronic diseases, we assessed the association between a posteriori identified DPs in the Seguimiento Universidad de Navarra (SUN) Project - a prospective cohort study in a Mediterranean country - and breast cancer (BC) risk. Design: DPs were ascertained through a principal component analysis based on 31 predefined food groups. BC cases were initially identified through self-report or, if deceased, from death certificates or by notification by the next kin. Women reporting BC were asked to provide a copy of their medical report and diagnoses for confirmation purposes. We fitted Cox regression models to assess the association between adherence to the identified DPs and BC risk. Setting: Spanish university graduates. Participants: We included 10 713 young and middle-aged - mainly premenopausal - women. Results: After a median follow-up of 10 center dot 3 years, we identified 100 confirmed and 168 probable incident BC cases. We described two major DPs: 'Western dietary pattern' (WDP) and 'Mediterranean dietary pattern' (MDP). A higher adherence to a WDP was associated with an increased risk of overall BC (multivariable-adjusted HR for confirmed BC Q4 v. Q1 1 center dot 70; 95 % CI 0 center dot 93, 3 center dot 12; P for trend = 0 center dot 045). Contrarily, adherence to a MDP was inversely associated with premenopausal BC (multivariable-adjusted HR Q4 v. Q1 0 center dot 33; 95 % CI 0 center dot 12, 0 center dot 91). No significant associations were observed for postmenopausal BC. Conclusions: Whereas a higher adherence to the WDP may increase the risk of BC, a higher adherence to the MDP may decrease the risk of premenopausal BC.