OBJECTIVE: We determined associations between diet soda consumption and risk of incident metabolic syndrome, its components, and type 2 diabetes in the Multi-Ethnic Study of Atherosclerosis. RESEARCH DESIGN AND METHODS: Diet soda consumption was assessed by food frequency questionnaire at baseline (2000-2002). Incident type 2 diabetes was identified at three follow-up examinations (2002-2003, 2004-2005, and 2005-2007) as fasting glucose >126 mg/dl, self-reported type 2 diabetes, or use of diabetes medication. Metabolic syndrome (and components) was defined by National Cholesterol Education Program Adult Treatment Panel III criteria. Hazard ratios (HRs) with 95% CI for type 2 diabetes, metabolic syndrome, and metabolic syndrome components were estimated, adjusting for demographic, lifestyle, and dietary confounders. RESULTS: At least daily consumption of diet soda was associated with a 36% greater relative risk of incident metabolic syndrome and a 67% greater relative risk of incident type 2 diabetes compared with nonconsumption (HR 1.36 [95% CI 1.11-1.66] for metabolic syndrome and 1.67 [1.27-2.20] for type 2 diabetes). Of metabolic syndrome components, only high waist circumference (men >or=102 cm and women >or=88 cm) and high fasting glucose (>or=100 mg/dl) were prospectively associated with diet soda consumption. Associations between diet soda consumption and type 2 diabetes were independent of baseline measures of adiposity or changes in these measures, whereas associations between diet soda and metabolic syndrome were not independent of these factors. CONCLUSIONS: Although these observational data cannot establish causality, consumption of diet soda at least daily was associated with significantly greater risks of select incident metabolic syndrome components and type 2 diabetes.