Finger nailfold capillary pressure (CP) was determined by direct microinjection technique in nine insulin-dependent male diabetic and nine matched control subjects. Mean CP, recorded under resting conditions after acclimatization in a constant temperature room at 24°C, was similar in all parts of the capillary loop in the two groups. Control subjects demonstrated a positive correlation between skin temperature and CP (r = + 0.74 and P < 0.01, arterial limb values; r = +0.61 and P < 0.01, summit and venous limb values). No such relationship was found for diabetics under the initial warm conditions of study (r = −0.15, arterial limb values; r = −0.44, summit and venous limb values). Mean arterial limb CP, recorded during postocclusive reactive hyperemia, was significantly higher in control subjects than in diabetics (P < 0.001). The diabetic group exhibited no clinical evidence of neuropathy, and their digital vasoconstrictor capacity appeared to be intact. These findings are interpreted as evidence for an intrinsic microvascular lesion at the precapillary sphincter level in insulin-dependent diabetic patients, which may result in the failure of vasodilatation to respond to physiologic Stress.