1. Microvascular blood flow responses to injury and capillary ultrastructure were assessed by laser Doppler flowmetry and detailed light and electron microscopy respectively in skin biopsied from 28 patients with insulin-dependent diabetes and 17 control subjects.

2. The hyperaemic response induced by biopsy (P < 0.001) and heating to 44°C (P < 0.001) was significantly lower in the diabetic patients and showed progressive impairment with the severity of complications (P < 0.001).

3. Skin capillary basement membrane thickness was significantly increased in the diabetic patients (P < 0.001) and also increased with the severity of complications (P < 0.002). Both the luminal area (P < 0.001) and the endothelial cell outer perimeter (P < 0.002), measures of luminal and capillary size, respectively, were significantly reduced in all diabetic patients.

4. Basement membrane thickness was related significantly to the impaired hyperaemic response to both biopsy (P < 0.01) and thermal injury (P < 0.01).

5. Our findings support the hypothesis that structural abnormalities, which are characterized by an early reduction in capillary size and later thickening of basement membrane, form an important mechanism for the impaired hyperaemic response in diabetic patients.

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