['The aim of this study was to evaluate the effect of acute glycemia increase on microvasculature and endothelium in Type 1 diabetes during hyperinsulinemic clamp.', 'Sixteen patients (51 ± 7\u2009yrs) without complications were examined during iso- and hyperglycemic clamp (glucose increase 5.5\u2009mmol·L(-1)). Insulin, lipid parameters, cell adhesion molecules and fibrinogen were analyzed. Microvascular reactivity (MVR) was measured by laser Doppler flowmetry.', 'Maximum perfusion and the velocity of perfusion increase during PORH were higher in hyperglycemia compared to baseline (47 ± 16 versus 40 ± 16\u2009PU, P < 0.01, and 10.4 ± 16.5 versus 2.6 ± 1.5\u2009PU·s(-1), P < 0.05, resp.). Time to the maximum perfusion during TH was shorter and velocity of perfusion increase during TH higher at hyperglycemia compared to isoglycemic phase (69 ± 15 versus 77 ± 16\u2009s, P < 0.05, and 1.4 ± 0.8 versus 1.2 ± 0.7\u2009PU·s(-1), P < 0.05, resp.). An inverse relationship was found between insulinemia and the time to maximum perfusion during PORH (r = -0.70, P = 0.007).'] Does acute hyperglycemia impair microvascular reactivity and endothelial function during hyperinsulinemic isoglycemic and hyperglycemic clamp in type 1 diabetic patients?