Rheological tests were performed in four rigorously defined groups of patients with Raynaud's phenomenon: primary family-related (n = 21), primary family-unrelated (n = 30), 'possible scleroderma' (n = 26), and scleroderma (n = 19). Whole blood and plasma viscosity, and hematocrit were significantly higher in the 'possible scleroderma' group. We conclude that the contribution of central rheological abnormalities in the pathogenesis of Raynaud's phenomenon is limited.