To determine whether gestational diabetes mellitus (GDM) increases the risk for preeclampsia independent of other risk factors.The association between GDM and preeclampsia was analyzed in a population of women who had given birth to singletons registered in Swedish Medical Birth Register from 1992 through 1996 (n=430,852).GDM occurred in 0.8% and preeclampsia in 2.9% of all pregnancies. The rate of preeclampsia was higher in the GDM than in the non-GDM group (6.1% versus 2.8%). High age, nullipara, chronic hypertension, kidney disease, and high body mass index (BMI) were all independently associated with increased risk for preeclampsia. Smoking was associated with decreased risk. Adjusted odds ratio for GDM as a risk factor for preeclampsia was 1.61 (95% confidence interval (CI) 1.39-1.86) when prepregnancy BMI, which was a true confounder, was included in the last step of the multiple logistic regression analysis.There is an independent and significant association between GDM and preeclampsia. Obesity is a major confounding factor but could not explain the total excess risk.