OBJECTIVE Whether dietary habits early in life can affect risk of type 2 diabetes (T2DM) in adulthood is unknown. We evaluated the relationship between dietary patterns during adolescence and risk of T2DM in midlife. RESEARCH DESIGN AND METHODS We examined the 7-year incidence of T2DM in relation to dietary patterns during high school among 37,038 participants in the Nurses' Health Study II cohort, who completed a food-frequency questionnaire about their diet during high school. Dietary patterns were derived by factor analysis. Cox proportional hazards regression was used to estimate relative risk (RR) and 95% CI. RESULTS The prudent pattern, characterized by healthy foods, was not associated with risk of T2DM. The Western pattern, characterized by desserts, processed meats, and refined grains, was associated with 29% greater risk of T2DM (RR 1.29; 95% CI 1.00-1.66; P trend 0.04), after adjusting for high school and adult risk factors comparing extreme quintiles, but was attenuated after adjusting for adult weight change (1.19; 0.92-1.54). Women who had high Western pattern scores in high school and adulthood had an elevated risk of T2DM compared with women who had consistent low scores (1.82; 1.35-2.45), and this association was partly mediated by adult BMI (1.15; 0.85-1.56). CONCLUSIONS A Western dietary pattern during adolescence may increase risk of T2DM in later life, partly through adult weight gain. Preventive measures should be aimed at developing healthy dietary habits that begin in early life and continue through adulthood.