To investigate the relationship between cardiovascular risk factors and the fluidity of blood, a random sample of the population consisting of 4022 persons ages 25 to 64 years was investigated for plasma viscosity, hemoglobin, and total serum protein. A total of 567 healthy nonsmokers and 287 healthy smokers were identified and compared. Plasma viscosity was found to be elevated in male smokers; this was related to both the degree and the duration of cigarette abuse. Plasma viscosity proved to be age-dependent in smokers, but did not change with age in nonsmokers. Total serum protein decreased with age in nonsmokers, while it did not change in smokers. Hemoglobin increased only in female smokers. These findings suggest that blood fluidity is jeopardized in smokers. In men the hemorheological deficit is mostly due to a rise of plasma viscosity, which, in turn, may be caused by an elevation of plasma fibrinogen levels. In women it is predominantly due to an increase in hemoglobin. These alterations in hemorheological variables may be a marker for increased cardiovascular risk in smokers and could reduce blood flow and hinder microcirculatory function.