OBJECTIVE: The objective of the study was to prospectively assess the association between lactation duration and incidence of the metabolic syndrome among women of reproductive age. RESEARCH DESIGN AND METHODS: Participants were 1,399 women (39% black, aged 18-30 years) in the Coronary Artery Risk Development in Young Adults (CARDIA) Study, an ongoing multicenter, population-based, prospective observational cohort study conducted in the U.S. Women were nulliparous and free of the metabolic syndrome at baseline (1985-1986) and before subsequent pregnancies, and reexamined 7, 10, 15, and/or 20 years after baseline. Incident metabolic syndrome case participants were identified according to National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III) criteria. Complementary log-log models estimated relative hazards of incident metabolic syndrome among time-dependent lactation duration categories by gestational diabetes mellitus (GDM) adjusted for age, race, study center, baseline covariates (BMI, metabolic syndrome components, education, smoking, physical activity), and time-dependent parity. RESULTS: Among 704 parous women (620 non-GDM, 84 GDM), there were 120 incident metabolic syndrome case participants in 9,993 person-years (overall incidence rate 12.0 per 1,000 person-years; 10.8 for non-GDM, 22.1 for GDM). Increased lactation duration was associated with lower crude metabolic syndrome incidence rates from 0-1 month through >9 months (P < 0.001). Fully adjusted relative hazards showed that risk reductions associated with longer lactation were stronger among GDM (relative hazard range 0.14-0.56; P = 0.03) than non-GDM groups (relative hazard range 0.44-0.61; P = 0.03). CONCLUSIONS: Longer duration of lactation was associated with lower incidence of the metabolic syndrome years after weaning among women with a history of GDM and without GDM, controlling for preconception measurements, BMI, and sociodemographic and lifestyle traits. Lactation may have persistent favorable effects on women's cardiometabolic health.