Direct intravital microscopic examinations of nailfold capillaries were made in two groups of subjects: 15 healthy volunteers (C) and 16 non-insulin dependent (D II) diabetic patients. In the diabetic group, the disease duration was less than 1 year (n = 4), between 1 and 10 years (n = 8) and between 10 and 18 years (n = 4). Capillary morphology was evaluated and the distribution of morphological patterns was significantly different between the two groups (P less than 0.001). The number of enlarged capillaries was increased in the D II group compared to the C group and capillaries with nodular apical elongations were only found in diabetics. Capillary blood flow velocity (CBFV) was measured during rest and after release of 60 s arterial occlusion. To assess autoregulatory capacity we determined peak CBFV post occlusion and time to reach it in single capillaries. Mean resting CBFV was not statistically different in the two groups but mean peak CBFV post occlusion was significantly lower (C: 1.49 +/- 0.14 mm s-1; mean +/- SE; D II: 0.93 +/- 0.13 mm s-1, P less than 0.05) and mean time to reach it significantly prolonged (C: 8.9 +/- 0.6 s; D II: 18.0 +/- 1.9 s; P less than 0.05) in diabetics compared to controls. Thus skin microvascular autoregulatory responses are disturbed in these patients. The impairments of the reactive hyperaemia response could not be correlated to either metabolic control or duration of the disease.