Hyperemic responses after arterial occlusion were investigated in patients with Raynaud's phenomenon due to systemic sclerosis (SSc) and in healthy controls. The hyperemia due to arterial occlusion for 2 min was measured by laser-Doppler flowmetry. If the hyperemic response was absent, the measurements were repeated after vasodilatation was induced by hand warming in warm water. Reactive hyperemia was absent in 12 patients when investigations were performed on the unwarmed hand, and these patients had very low resting blood flows. After vasodilatation was induced in these patients, and also in those patients whose resting blood flow was normal, hyperemic responses were comparable in magnitude with those in the controls. The slope of the hyperemic response was significantly less in the patients (1.11 +/- 0.02 V/s) than in the controls (1.28 +/- 0.03 V/s), and therefore, the time course of the hyperemia was lengthened in the patients with SSc, with a delay to achieving maximum blood flow of 2 min. Peak blood flow was directly related to the level of the initial blood flow. These findings support the view that reactive hyperemia is principally a mechanical phenomenon, and also that vessel wall reactivity is abnormal in SSc, producing delayed hyperemic responses. The magnitude of the hyperemia depends on initial flow rates, and the apparent lack of these responses in SSc is a result of their low, but reversible, resting blood flow.