The early preclinical detection of cerebrovascular complications in individuals with diabetes is one of the goals of care described in the St. Vincent Declaration. In accordance with this goal, the aim of the present work was to investigate whether altered cerebral microvascular function in patients suffering from type 1 diabetes can be detected with a transcranial Doppler probe after the administration of acetazolamide. A total of 72 type 1 diabetic patients and 40 healthy control subjects entered the study. Patients were divided into two groups: those with long-term diabetes (disease duration of >10 years, n = 37) and those with short-term diabetes (disease duration of ≤10 years, n = 35). Mean blood-flow velocity in the middle cerebral artery (MCAV) was measured at rest and at 5, 10, 15, and 20 min after intravenous administration of 1 g acetazolamide with a transcranial Doppler probe and expressed as the percentage change from the pretest measurement. The percentage increase in MCAV (cerebrovascular reactivity) was calculated at each time point and compared between the groups. Cerebrovascular reserve capacity (CRC), expressed as the maximal percentage increase of the MCAV, was compared between the groups. Additionally, a reproducibility study of CRC was performed in 10 patients, using intra-class correlations. Cerebrovascular reactivity in the long-term diabetes group was lower (means ± SD: 5 min, 23.4 ± 15.4%; 10 min, 28.8 ± 17.0%; 15 min, 30.0 ± 15.6%; 20 min, 24.2 ± 17.8%) than that of the control subjects (5 min, 43.5 ± 23.9%; 10 min, 55.3 ± 24.0%; 15 min, 56.7 ± 23.8%; 20 min, 54.8 ± 25.9%) and the short-term diabetic patients (5 min, 43.6 ± 25.9%; 10 min, 52.2 ± 27.7%; 15 min, 55.3 ± 32.2%; 20 min, 45.8 ± 35.8%). CRC was lower in the long-term diabetes group than in the control group or the short-term diabetes group. Impairment of cerebrovascular reactivity was associated with retino- and nephropathy and increased levels of fibrinogen. In contrast, CRC was independent from actual glucose, insulin, glycosylated hemoglobin, von Willebrand factor antigen, and α-2 macroglobulin levels. Transcranial Doppler measurements of the changes in MCAV after stimulation with acetazolamide can detect altered cerebral microvascular function in patients with diabetes. Cerebrovascular reactivity and reserve capacity are reduced in patients with long-term diabetes. Further prospective studies should delineate the clinical significance of our results.