Objectives We sought to determine if the rate of preeclampsia is related to the severity of gestational diabetes mellitus (GDM), and if it can be decreased by optimizing glycemic control. Study design A retrospective analysis of prospectively collective data of 1813 patients with GDM was performed to determine the rate of preeclampsia. Patients were stratified after treatment was begun by level of glycemic control (well controlled was defined as mean blood glucose < 95 mg/dL). The extent of hyperglycemia was analyzed by the level of the abnormality in the oral GTT and by the degree of abnormality of daily glucose control after treatment has begun. Severity of GDM was categorized using fasting plasma glucose (FPG) on a 3-hour oral GTT by 10 mg/dL increments. Results Overall, preeclampsia was diagnosed in 9.6%(174/1813) of diabetic patients. The GDM subjects who developed preeclampsia were significantly younger, had a higher nulliparity rate, were more obese, and gained significantly more weight during pregnancy. However, no difference was found in glycemic profile characteristics between the 2 groups. A comparison between patients with FPG <105 and FPG >105 revealed that the rate of preeclampsia increased significantly, 7.8%vs 13.8%, (O.R 1.81, 95%CI 1.3-2.51). For GDM patients with only mild hyperglycemia (FPG <105 mg/dL), no significant difference was found in the rate of preeclampsia. Preeclampsia rate was further evaluated in relation to level of glycemic control; for the wellcontrolled patients (mean blood glucose [MBG] <95 mg/dL, n=994), similar rates of preeclampsia were found between each category of FPG severity. In contrast, in poorly controlled patients (MBG >95 mg/dL, n=819), a comparison between severity threshold of FPG <115 and FPG >115 revealed that the preeclampsia rate was 9.8%vs 18%(O.R 2.56, 95%C.I. 1.5-4.3). In a logistic regression model, only prepregnancy BMI (O.R 2.3, 95%CI 1.16-2.30) and severity of GDM (O.R 1.7, 95%CI 1.21-2.38) were independently and significantly associated with an increased risk of preeclampsia. Conclusion The rate of preeclampsia is influenced by the severity of GDM and prepregnancy BMI. Optimizing glucose control during pregnancy may decrease the rate of preeclampsia, even in those with a greater severity of GDM. Objectives We sought to determine if rate of preeclampsia is related to the severity of gestational diabetes mellitus (GDM), and if it can be decreased by optimizing glycemic control. Study design A retrospective analysis of prospectively collective data of 1813 patients with GDM was performed to determine the rate of preeclampsia. Patients were stratified after treatment was begun by level of glycemic control (well controlled was defined as mean blood glucose <95 mg / dL). The extent of hyperglycemia was analyzed by the level of the abnormality in the oral GTT and by the degree of abnormality of daily glucose control after treatment has begun. Severity of GDM was categorized using fasting plasma glucose (FPG) on a 3-hour oral GTT by 10 mg / dL increments. Results Overall, preeclampsia was diagnosed in 9.6 % (174/1813) of diabetic patients. The GDM subjects who developed preeclampsia were significantly younger, had a higher nulliparity rate, were more obese, and increased significantly mor A comparison between patients with FPG <105 and FPG> 105 revealed that the rate of preeclampsia increased significantly, 7.8% vs 13.8%, (OR 1.81 , 95% CI 1.3-2.51). For GDM patients with only mild hyperglycemia (FPG <105 mg / dL), no significant difference was found in the rate of preeclampsia. Preeclampsia rate was further evaluated in relation to level of glycemic control; for In contrast, in poorly controlled patients (MBG> 95 mg / dL, n = 994), similar rates of preeclampsia were found between each category of FPG severity. = 819), a comparison between severity threshold of FPG <115 and FPG> 115 revealed that the preeclampsia rate was 9.8% vs 18% (OR 2.56, 95% CI 1.5-4.3). In a logistic regression model, only prepregnancy BMI ( OR 2.3, 95% CI 1.16-2.30) and severity of GDM (OR 1.7, 95% CI 1.21-2.38) were independently and significantly associated with an increased risk of preeclampsia. Conclusion The rate of preeclampsia is influenced by the severity of GDM and prepregnancy BMI. Optimizing glucose control during pregnancy may decrease the rate of preeclampsia, even in those with a greater severity of GDM.