The aim was to To study the relationship between BMI and hospitalization for heart failure in people with Type 2 diabetes. We identified 83 021 individuals with Type 2 diabetes from the Swedish National Diabetes Registry during 1998-2003, who were followed until hospitalization for heart failure, death or end of follow-up on 31 December 2009. Cox regression analyses were performed, adjusting for age, sex, HbA(1c), blood pressure, diabetes duration, smoking, microalbuminuria, cardiac co-morbidities, glucose-lowering and anti-hypertensive medications. During a median follow-up of 7.2 years, 10 969 patients (13.2%) were hospitalized with heart failure. By categories of BMI, with BMI 20 to < 25 kg/m(2) as the reference, hazard ratios for patients during follow-up were 1.07 (95% CI 0.91-1.26) for a mean BMI of < 20 kg/m(2), 1.04 (95% CI 0.98-1.11) for BMI 25 to < 27.5 kg/m(2), 1.22 (95% CI 1.15-1.30) for BMI 27.5 to < 30 kg/m(2), 1.54 (95% CI 1.45-1.63) for BMI 30 to < 35 kg/m(2), 2.16 (95% CI 2.00-2.33) for BMI 35 to < 40 kg/m(2) and 3.22 (95% CI 2.88-3.60) for BMI 40 kg/m(2) or higher. There was a significant interaction between BMI and sex (P = 0.0006), with numerically higher hazard ratios for hospitalization for heart failure within each BMI category for men than for women. Obesity is strongly related to hospitalization for heart failure in people with Type 2 diabetes, and the relationship is somewhat stronger for men than for women. Preventing weight gain and promoting weight loss may be crucial in reducing the incidence of future hospitalizations for heart failure in this population.