While recent studies suggested that both general adiposity and abdominal adiposity are associated with the risk of death and support the use of waist circumference or waist-to-hip ratio (WHR) in addition to body index mass (BMI) in assessing the risk of death, this issue remains cintroversial since most authors conclude that BMI explains almost all the obesity-related risk of diabetes and conary heart disease (CHD). We investigated the separate effects of BMI and WHR on blood rheology in 430 patients attending to a metabolic check-up and exhibiting all the spectrum of age (11-77 yr) and BMI (15-50 kg/m2). BMI and WHR are correlated to each other (r = 0.269; p = 0.009) and are both predictors of blood viscosity (BMI: r = 0.15516; p = 0.004; WHR: r = 0.3638; p = 0.03). However while looking at determinants of viscosity these correlations had not the same meaning. For BMI it was explained by its correlation with plasma viscosity (r = 0.17718; p = 0.00105) and red blood cells (RBC) aggregation (all Myrenne and SEFAM indices with r ranging between 0.226 and 0.430) while these parameters were not correlated to WHR. By contrast WHR was strongly correlated with hematocrit (r = 0.524; p = 0.0003) which was not correlated with BMI. A forward stepwise regression selected WRH as a better predictor of blood viscosity, excluding BMI. Thus both BMI and WHR are associated with increased blood viscosity but these correlations reflect separate mechanisms. These data suggest that both overall adiposity and abdominal adiposity induce hyper viscosity, consistent with epidemiological studies linking the risk of CHD to abdominal adiposity and BMI.