Some workers regard whole-blood and plasma viscosity as provoking factors in Raynaud's disease. There was no significant difference, however, in whole-blood and plasma viscosity between ten young women with primary Raynaud's phenomenon and ten age and sex matched controls. Venesection, which reduces plasma viscosity, did not change the ease with which Raynaud's phenomenon was provoked in five patients. Increased viscosity might be a causal factor in secondary forms of Raynaud's disease, but it does not seem to be an important factor in primary Raynaud's disease.