Context: Diabetes and obesity, components of the metabolic syndrome, are common characteristics of women with prior gestational diabetes mellitus (GDM). Due to increasing incidence of diabetes and obesity, the metabolic syndrome might comprise a major health problem among these women. Objective: The objective was to estimate the prevalence of the metabolic syndrome by three different criteria [World Health Organization 1999 (WHO), The National Cholesterol Education Program Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults 2001, and European Group for the Study of Insulin Resistance 2002] among women with previous GDM. Design: We conducted a follow-up study of a Danish cohort of women admitted in 1978-1996 to the Diabetes and Pregnancy Center, Rigshospitalet, Copenhagen University Hospital, with diet-treated GDM. The follow-up took place in 2000-2002 at median 9.8 yr (interquartile range 6.4-17.2) after pregnancy. Results were compared with a control group of 1000 age-matched women from a population-based sample (Inter99). Participants: Four hundred eighty-one women at median age 43 yr (interquartile range 38-48) participated. Main Outcome Measures: The main outcome measures were body mass index (BMI), glucose tolerance, blood pressure, lipid profile, and insulin resistance. Results: Independent of the criteria, the prevalence of the metabolic syndrome was three times higher in the prior GDM group, compared with the control group (e.g. WHO: 38.4 vs. 13.4%, P < 0.0005). Ageand BMI-adjusted odds ratio for having the WHO-defined metabolic syndrome was 3.4 (95% confidence interval 2.5-4.8) for the priorGDM group vs. the control group. Obese women (BMI > 30 kg/m2) with previous GDM had a more than 7-fold increased prevalence of the metabolic syndrome (WHO), compared with normal-weight prior GDM women (BMI < 25 kg/m2). In glucose-tolerant women, the prevalence was doubled in the prior GDM group, compared with control group. Conclusion: The prevalence of the metabolic syndrome was three times as high in women with prior diet-treated GDM, compared with age-matched control subjects.