Whole-blood viscosity (WBV) depends on vascular geometry and blood physiological constituents. Diabetes mellitus, hypertension, dyslipidemia and obesity - the major components of metabolic syndrome (MetS) - can independently affect blood vessels and microcirculation. MetS is the state of oxidative stress and systemic inflammation. Pro-oxidant and inflammatory cytokines induce endothelial dysfunction. Morphological alterations of erythrocytes could be a consequence of decreased erythrocytes deformability, oxidative stress and systemic inflammation. These events altogether lead to increased WBV. In this review, the effect of WBV in different components of the MetS and WBV with regard to oxidative stress and inflammation - common states in chronic disease - are discussed. � 2012 Future Medicine Ltd.