ObjectiveTo identify if gestational diabetes mellitus (GDM) is a clinically useful marker of future cardiovascular disease (CVD) risk and if GDM combined with other risks (smoking, hypertension or body mass) identifies high-risk groups. DesignPopulation-based matched case-control study. SettingNational Swedish register data from 1991 to 2008. PopulationA total of 2639 women with a cardiovascular event and matched controls. MethodsConditional logistic regression examined associations with CVD before and after adjustment for conventional risk factors and confounders. Effect modification for the association of GDM with CVD by body mass index (BMI), smoking and chronic hypertension was assessed by stratification and interaction testing. Adjustment for diabetes post-pregnancy evaluated its mediating role. Main outcome measuresInpatient diagnoses or causes of death identifying ischemic heart disease, ischemic stroke, atherosclerosis or peripheral vascular disease. ResultsThe adjusted odds ratios (and 95% confidence intervals) for the association of CVD with GDM are 1.51 (1.07-2.14), 2.23 (2.01-2.48) for smoking, 1.98 (1.71-2.29) for obesity and 5.10 (3.18-8.18) for chronic hypertension. In stratified analysis the association of CVD with GDM was only seen among women with BMI 25, with an odds ratio of 2.39 (1.39-4.10), but only women with a BMI