Objective: It is controversial, if subclinical hypothyroidism increases cardiovascular risk. Plasma viscosity is a hemorheological parameter, which is accepted as an early cardiovascular risk factor. We investigated the alterations in plasma viscosity in women with subclinical hypothyroidism. Design: 40 female patients with subclinical hypothyroidism and 31 age- and weight-matched healthy women were included. Free thyroxine (FT4), thyroid stimulating hormone (TSH), lipid parameters, fibrinogen, C-reactive protein (CRP) levels, hematocrit and plasma viscosity were measured in all subjects. Main outcome: Plasma viscosity, total cholesterol and low density lipoprotein were significantly increased and high density lipoprotein was significantly decreased in patients with subclinical hypothyroidism. No significant correlation was found among the parameters. Conclusion: Increased plasma viscosity in patients' group suggests that cardiovascular risk might be increased in patients with subclinical hypothyroidism. As far as we could reach, this is the first study concerning plasma viscosity in subclinical hypothyroidism.