We examined whether hemorheology and platelet function are affected in essential hypertensives (EHTs) of the World Health Organization stage I when complicated with metabolic syndrome (Mets). In 156 untreated EHTs, blood viscosity and platelet surface markers were determined. Blood viscosity was significantly elevated in 54 subjects with Mets compared with 102 subjects without Mets. Hematocrit and plasma viscosity increased in the group with Mets, although red blood cell rigidity index "k" did not differ between groups. As a whole group, blood viscosity correlated positively with hematocrit and plasma viscosity. Additionally, plasma viscosity correlated positively with plasma leptin, triglyceride, homeostasis model assessment index, C-reactive protein, and plasma fibrinogen, but negatively with high-density lipoprotein cholesterol. In contrast, no differences were seen in platelet surface markers between groups. In conclusion, EHTs of the early stage complicated with Mets are characterized by increased blood viscosity due to hemoconcentration and increased plasma viscosity.