The effect of local temperature versus sympathetic tone on digital perfusion in Raynaud's phenomenon. Matched groups of 7 to 8 patients with primary Raynaud's phenomenon, systemic sclerosis, and undifferentiated connective tissue disease and cold-tolerant normal control subjects were studied by simultaneous digital strain gauge plethysmography and laser Doppler capillary velocimetry during two controlled cycles of hand warming and cooling with and without addition of central cooling and during clinical maneuvers to evoke sympathetic tone. Transient vasoconstrictor responses of comparable degree could be evoked in all patient groups and in both the arterial and microvascular beds. While the addition of central cooling had little influence on arterial flow, patients with systemic sclerosis manifested a failure to maintain nutritive perfusion at finger temperatures associated with Raynaud's phenomenon. Linear regression and multivariate analysis suggested that finger temperature was the principal determinant of arterial flow in systemic sclerosis and that arterial flow was the principal determinant of microvascular perfusion. The inability of patients with systemic sclerosis to maintain nutritive flow in the face of either reflex or cold-induced proximal arterial constriction is consistent with their clinical propensity to ischemic tissue injury and separates these patients physiologically from other forms of Raynaud's phenomenon. 