Carbohydrate and lipid metabolism was cross-sectionally assessed in 16 patients with endogenous hypercortisolism (endogenous Cushing syndrome). Five patients (31%) had fasting glucose levels over 6.6mmol/l and a HbA1C over 7.5%. Six patients (38%) had diabetes mellitus based on an abnormal 75g oral glucose tolerance test (OGTT) and two additional patients (13%) had impaired glucose tolerance based on an OGTT. Compared to obese individuals, patients with Cushing syndrome had an elevated glucose but no elevated insulin response to the OGTT. Regression analysis showed positive correlations between 24-h urinary free cortisol (UFC) and fasting blood glucose (P<0.0005), UFC and OGTT glucose area under the curve (AUC) (P<0.01), and UFC and HbAlc (P<0.005). UFC levels were negatively correlated (P<0.05) with OGTT insulin AUC and insulin/glucose ratios. Eleven (69%) patients required anti-hypertensive therapy for blood pressure control. Total cholesterol and triglycerides were elevated in patients with Cushing syndrome compared to obese controls, while LDL and HDL cholesterol, and Lp(a) were similar in the two groups. We conclude that impaired glucose tolerance and/or diabetes in patients with endogenous Cushing syndrome is due to the hyperglycemic effects of cortisol with relative insulinopenia. Thus, Cushing syndrome shares features with both the Metabolic Syndrome X and NIDDM, including impaired glucose uptake, hyperlipidemia and hypertension. However, in Cushing syndrome, a relative insulinopenia occurs, while in Metabolic Syndrome X and NIDDM, insulin excess is observed. In Cushing syndrome, as the hypercortisolemia exacerbates, insulinopenia becomes more paramount, suggesting that cortisol exerts a direct or indirect “toxic” effect on the β-cell.