The possible relationship among blood rheology pattern, renal hemodynamics and proteinuria was investigated in 22 long-term insulin-dependent diabetic patients with overt nephropathy. Higher blood, plasma and serum viscosity and lower erythrocyte filtrability were found in our patients with overt nephropathy, than in patients without renal microvascular complications. Several negative correlations between blood viscosity and renal hemodynamic parameters (i.e., glomerular filtration rate and renal plasma flow) were demonstrated in the diabetics with overt nephropathy. Furthermore increased plasma fibrinogen, fibronectin and acute-phase protein levels were found in diabetics with overt nephropathy, compared to diabetics without renal changes. The data may suggest a pathogenetic role for blood rheology changes in the progression of diabetic glomerular complications.