The microvascular response of foot skin to minor thermal injury and the skin of the anterior abdominal wall to injury from a needle was assessed by laser Doppler flowmetry in 23 patients with type I diabetes and 21 healthy control subjects. After minor thermal injury mean (SD) maximum skin blood flow was significantly lower in the diabetic group than the control group (0.53 (0.11) v 0.72 (0.10) V, in arbitrary units of flow, respectively, p less than 0.001) and was negatively correlated with the duration of diabetes (r = -0.60; p less than 0.01). After needle injury a similar pattern of impairment was seen, the peak flow value recorded being significantly lower in the diabetic group than the control group (0.28 (0.10) v 0.41 (0.09) V, respectively; p less than 0.001) and also negatively correlated with the duration of diabetes (r = -0.61; p less than 0.01). There was a significant relation between the response obtained at the two sites of injury in the diabetic group (r = +0.72, p less than 0.001) but not in the control group. The impairment in response was not related to diabetic control and was not explicable in terms of a reduction in superficial skin capillary density. The inability of the diabetic skin microvasculature to respond normally to injury may be an important factor in the development of foot ulceration that often follows minor trauma.