Obesity is known to be associated with an elevated risk for cardiovascular disease. Rheological disturbances could be factors contributing to these vascular complications. Therefore we have evaluated blood viscosity parameters in 128 obese (BMI greater than 28 kg/m2) adults and in 90 non-obese healthy subjects. Whole blood, plasma and erythrocyte viscosity values were determined with a Contraves LS30 viscosimeter. Plasma and whole blood viscosity were significantly (all P less than 0.001) increased in the obese subjects. The increased low shear erythrocyte viscosity suggested a diminished erythrocyte deformability in obesity. The rheological abnormalities were present even in the absence of impaired glucose tolerance, diabetes, hypertension or hyperlipidaemia. In the obese group the rheological parameters showed significant correlations (at least P less than 0.05) with BMI, insulin or C-peptide area during an oral glucose tolerance test and plasma lipids. Our findings demonstrate that obesity per se may be associated with abnormal blood viscosity properties.