Plasma viscosity is a major determinant of capillary blood flow. It has been suggested that alteration in plasma viscosity contributes to impaired blood flow and to increased cardiovascular risk. The aim of this study was to investigate the plasma viscosity levels and its possible role in the cardiovascular risk in patients with low grade nephrotic proteinuria. 20 patients with low-grade nephrotic proteinuria (mean age: 35+/-5 years) and 20 healthy controls (mean age: 33+/-4 years) were participated in the study. Plasma viscosity was measured by Harkness capillary viscometer. Biochemical analysis were measured by commercial enzymatic kits. Plasma viscosity, plasma levels of creatinine, fibrinogen and triglyceride were increased in patients with proteinuria than in the healthy controls (p<0.001, p<0.001, p<0.001, and p<0.001, respectively). The plasma levels of total protein and albumin were significantly lower in patients with low grade nephrotic proteinuria than in healthy controls (p<0.001 and p<0.001, respectively). Plasma viscosity was negatively correlated with plasma albumin (r=-0.835, p<0.001) and total protein (r=-0.862, p<0.001) in proteinuric patients. When the correlation analyses were performed a significant positive correlation was found between plasma viscosity and fibrinogen (r=0.636, p<0.001). In the stepwise multiple regression analysis plasma viscosity was found to be related with plasma total protein (t=-6.456, p<0.001) in the patients. When the stepwise multiple regression analysis were performed in healthy controls, the significant relationship was only found between plasma viscosity and fibrinogen (t=+2.202, p<0.01). These results suggested that altered plasma composition associated with low-grade nephrotic proteinuria may be involving the determination of plasma viscosity. Thus, the plasma viscosity in patients with low-grade nephrotic proteinuria may have a prognostic value in assessing cardiovascular risk in this group.