This report is an attempt to gain more complete knowledge of the causes of the tendency to thrombosis in the nephrotic syndrome. Our purpose was to detect some rheological abnormalities in a group of nephrotic patients having normal renal function. Plasma and blood viscosity, erythrocyte filtration flow (EFF) and some indices of the carbohydrate metabolic state (glycosylated hemoglobin Ale; HbAlc, fasting glycemia and oral glucose tolerance test) were investigated in 10 patients with newly diagnosed primary nephrotic syndrome and 10 healthy subjects. EFF was determined in order to avoid variables depending upon plasma, white cells and hematocrit. The mean level of HbAlc was higher in the nephrotic group, in which we found 4 patients with impaired glucose tolerance according to the criteria of the National Diabetes Data Group. EFF was lower in nephrotic patients than in controls and correlated inversely with HbAlc. These findings indicate that nephrotic patients have a reduced red cell deformability which seems to be related to the abnormal carbohydrate metabolism commonly observed in this syndrome.