BACKGROUND: Cardiovascular events (stroke or myocardial infarction) are often associated with poorer prognosis in younger, compared with older individuals. We examined the associations between prepregnancy obesity and the risks of myocardial infarction and stroke in young, healthy women. METHODS AND RESULTS: All Danish women giving birth during 2004-2009 without a history of renal disease or cardiovascular disease were identified from national registers and followed for a median time of 4.5 years (interquartile range, 2.8-5.8). They were grouped according to prepregnancy body mass index (BMI) in underweight (BMI<18.5 kg/m(2)), normal weight (BMI=18.5-<25 kg/m(2)), overweight (BMI=25-<30 kg/m(2)), and obese (BMI≥30 kg/m(2)). The hazard ratios of myocardial infarction, ischemic stroke, and a composite outcome (myocardial infarction, stroke, cardiovascular death) were assessed using multivariable Cox regression models. We included 273 101 women with a median age of 30.4 years (interquartile range, 27.2-33.8). A total of 68 women experienced a myocardial infarction, and 175 women experienced an ischemic stroke. The adjusted hazard ratios of myocardial infarction compared with normal weight were 2.50 (95% confidence interval [95% CI], 0.97-6.50) in underweight, 1.68 (95% CI, 0.92-3.06) in overweight, and 2.63 (95% CI, 1.41-4.91) in obese women. For ischemic stroke the adjusted hazard ratios were 1.06 (95% CI, 0.44-2.28) in underweight, 1.27 (95% CI, 0.87-1.85) in overweight, and 1.89 (95% CI, 1.25-2.84) in obese women, respectively. For the composite outcome, hazard ratios were 1.34 (95% CI, 0.81-2.20), 1.43 (95% CI, 1.11-1.84), and 1.76 (95% CI, 1.31-2.34) for underweight, overweight, and obese women. CONCLUSIONS: In apparently healthy women of fertile age, prepregnancy obesity was associated with increased risks of ischemic stroke and myocardial infarction in the years after childbirth.