High blood glucose levels for several years is the major factor in the development and progression of microvascular complications in IDDM. Reducing mean blood glucose reduces the risk of progression of diabetic microvascular complications substantially. A curve-linear relationship exists between HbA1c levels and progression of diabetic retinopathy. Recent evidence also points to a close relationship between high blood glucose levels and progression of microvascular complications in NIDDM. The relationship between mean blood glucose and cardiovascular disease in diabetes has been unclear. Recent population-based studies give evidence for a linear association of glycemic control with the risk for cardiovascular disease in patients with NIDDM. However, randomized studies comparing different degrees of glycemic control in NIDDM and their impact on cardiovascular morbidity and mortality are urgently needed.