Fingertip skin blood flow was measured by laser Doppler flowmetry (as LDflux) under environmental conditions promoting vasodilation in Scottish patients with diabetes mellitus and Indian patients with leprosy. The reflex control of fingertip blood flow was assessed by measuring the reduction in LDflux induced by deep inspiratory gasp (IG) and cold challenge (CC) of immersing the contralateral hand in cold water. The uncomplicated diabetic patients showed normal vasomotor reflexes and an increased, though non significant, LDflux level (p <0.06). The patients with diabetic neuropathy had resting LDflux levels significantly less than the uncomplicated group and also had substantial impairment of both IG and CC reflexes. Those with retinopathy (but no clinically apparent neuropathy) had LDflux within the normal range, but they showed minor evidence of impairment of the vasomotor reflexes. The uncomplicated newly registered leprosy patients had reduced LDflux and substantial impairment of CC reflexes. These changes were more marked in newly registered leprosy patients with clinical evidence of neuropathy. Leprosy patients with long-standing neuropathy requiring orthopaedic treatment had LDfluxes so greatly reduced that measurement of vasomotor reflexes was not practicable. The CC reflex was more severely affected than the IG reflex and more frequently absent in leprosy patients, possibly because of associated sensory neuropathy affecting the afferent limb of this response. Thus laser Doppler flowmetry can detect impairment of reflex control of fingertip blood flow in both diabetes mellitus and leprosy, but there are functional differences in the pattern of autonomic impairment between the diseases, suggesting differences in the pathogenesis of nerve damage.