OBJECTIVE Increased Hb and ferritin have been associated with gestational diabetes mellitus (GDM). This study was performed to determine whether the prevalence of GDM is influenced by iron deficiency anemia. RESEARCH DESIGN AND METHODS In a retrospective case-control study, 242 women with iron deficiency anemia (Hb<10 g/dl with features of iron deficiency) were compared with 484 nonanemic women matched for year of birth, who were delivered within the same 24-month period in our hospital, with respect to maternal demographics, infant outcome, and the prevalence of GDM diagnosed according to the World Health Organization criteria. RESULTS There was no difference in the prepregnancy weight or BMI, but the anemic group had more multiparas and significantly lower gestational weight and BMI increments and prevalence of GDM (odds ratio [OR] 0.52, 95% CI 0.27-0.97), which was inversely correlated (P=0.045) with the duration of anemia. To determine the independent effect of anemia on GDM, multiple logistic regression analysis was performed adjusting for the effects of multiparity and BMI, and anemia was confirmed to be significantly associated with decreased prevalence of GDM (adjusted OR 0.46, 95% CI 0.23-0.90). CONCLUSIONS The prevalence of GDM is reduced in iron deficiency anemia, which probably served as a surrogate for other factors such as nutritional inadequacy and reduced gestational weight gain. Further studies on the relationship between nutritional improvement and the increasing prevalence of GDM in the developing world are warranted.