The relationship between either an acute or a chronic hyperglycemia and functional microcirculatory disturbances was studied in insulin‐dependent diabetic (IDD) patients in comparison to healthy volunteers. Acute hyperglycemia, provoked in 10 IDD patients, was accompanied by an increase in laser doppler skin blood flux while transcutaneous oxygen pressure (TcPO
$_2$
) decreased. These changes, accompanied by that in the concentration of moving blood cells indicate that acute hyperglycemia results in a vasodilation in favour of non‐nutritive microvascular shunts. The effect of chronic hyperglycemia was studied in 36 IDD patients who had a duration of diabetes of less than 5 years and had no clinical signs of micro‐ and macroangiopathy. In these patients, erythrocyte aggregation, plasma viscosity and fibrinogen concentration were increased and transcutaneous oxygen pressure reduced, compared with the levels seen in healthy subjects. More marked impairments were observed in patients with poor glycemic control. This suggests that chronic hyperglycemia involves functional disturbances which will contribute to the development of the vascular complications of diabetes. In 34 patients with poorly controlled diabetes who received intensive insulin therapy for 36 months, these changes were reversed in patients in whom good glycemic control was achieved within 2 months, but not in those in whom glycemic control remained poor. It is concluded that disturbances in blood flow and TcPO
$_2$
occur early in diabetes, and are consequences of poor glycemic control. These disturbances can be reversed or normalized if glycemic control is improved by intensive treatment.
