Background: Diabetes in patients with heart failure or myocardial infarction (MI) increases morbidity and mortality, but little is known about the impact of obesity on the risk of developing diabetes in these populations. Design: A cohort of patients consecutively hospitalized with heart failure (n¿=¿3472) or MI (n¿=¿5723) was followed in the period 1995-2006. Methods: Multivariable Cox proportional-hazard models were used to estimate the risk of developing diabetes according to the World Health Organization body mass index (BMI) classification. Normal weight patients (BMI 18.5-24.9¿kg/m(2)) were used as the reference. Results: In both populations, more than half of the patients with a BMI above 34.9¿kg/m(2) developed diabetes. In heart failure patients, a BMI above 24.9¿kg/m(2) was associated with an increased risk of diabetes for the three BMI groups, i.e. 25.0-29.9¿kg/m(2), 30.9-34.9¿kg/m(2), and >34.9¿kg/m(2), with adjusted hazard ratios (HRs) of 2.16 (95% confidence interval 1.50-3.12), 3.89 (2.61-5.78), and 6.06 (3.79-9.69), respectively. In MI patients, the adjusted HRs in the three corresponding BMI groups were 1.84 (1.44-2.37), 4.31 (3.26-5.69), and 9.50 (6.70-13.46), respectively. Incident diabetes was associated with increased cardiovascular and all-cause mortality risks with adjusted HRs of greater magnitude than in prevalent diabetes. Conclusion: BMI was an independent predictor of incident diabetes in patients with either heart failure or MI. More than half of the patients with a BMI above 34.9¿kg/m(2) developed diabetes during follow-up. Incident diabetes carries an increased mortality risk.