BACKGROUND:The incidence of heart failure (HF) is decreasing in older ages, but increasing rates have been observed among younger persons in Sweden. Therefore, we investigated the relationship between risk of hospitalization for HF and body mass index (BMI). METHODS:This was a prospective registry-based cohort study. We included 1,374,031 women aged 18-45 years (mean age 27.9 years) who gave birth during 1982-2014, and were registered in the Medical Birth Register. Information on hospitalization because of HF was collected through linkage to the National Inpatient Register. RESULTS:Compared to women with a BMI of 20-<22.5 kg/m2, women with a BMI of 22.5-<25.0 had a hazard ratio (HR) of 1.24 (95% confidence interval (CI), 1.10-1.39) for HF after adjustment for age, year, parity, baseline disorders, smoking, and education. The HR (95% CI) increased to 1.56 (1.36-1.78), 2.39 (2.05-2.78), 2.82 (2.43-3.28), and 4.51 (3.63-5.61) in women with a BMI of 25-<27.5, 27.5-<30, 30-<35, and ≥35 kg/m2, respectively. The multiple-adjusted HRs (95% CI) associated with risk of HF per one-unit increase in BMI in women with a BMI ≥ 22.5 kg/m2 ranged from 1.01 (0.97-1.06) for HF related to valvular disease to 1.14 (1.12-1.15) for coronary heart disease, diabetes, or hypertension. CONCLUSION:Increasing body weight was strongly associated with the risk of early HF in women. Compared with lean women, the risk for HF started to increase at high-normal BMI levels, and was nearly five-fold in women with a BMI ≥ 35 kg/m2.