OBJECTIVE-To examine whether the duration of abdominal obesity determined prospectively using measured waist circumference (WC) is associated with the development of newonset diabetes independent of the degree of abdominal adiposity. RESEARCH DESIGN AND METHODS-The Coronary Artery Risk Development in Young Adults Study is a multicenter, community-based, longitudinal cohort study of 5,115 white and black adults aged 18-30 years in 1985 to 1986. Years spent abdominally obese were calculated for participants without abdominal obesity (WC > 102 cm in men and >88 cm in women) or diabetes at baseline (n = 4,092) and was based upon repeat measurements conducted 2, 5, 7, 10, 15, 20, and 25 years later. RESULTS-Over 25 years, 392 participants developed incident diabetes. Overall, following adjustment for demographics, family history of diabetes, study center, and time varying WC, energy intake, physical activity, smoking, and alcohol, each additional year of abdominal obesity was associated with a 4% higher risk of developing diabetes [hazard ratio (HR) 1.04 (95% CI 1.02-1.07)]. However, a quadratic model best represented the data. HRs for 0, 1-5, 6-10, 11-15, 16-20, and > 20 years of abdominal obesity were 1.00 (referent), 2.06 (1.43-2.98), 3.45 (2.28-5.22), 3.43 (2.28-5.22), 2.80 (1.73-4.54), and 2.91 (1.60-5.29), respectively; P-quadratic < 0.001. CONCLUSIONS-Longer duration of abdominal obesity was associated with substantially higher risk for diabetes independent of the degree of abdominal adiposity. Preventing or at least delaying the onset of abdominal obesity in young adulthood may lower the risk of developing diabetes through middle age. © 2013 by the American Diabetes Association.