In renal allograft recipients who have developed occlusive arterial disease a significant elevation is seen in blood viscosity factors (plasma viscosity and aggregation of red cells) and biochemical factors (cholesterol, triglycerides and fibrinogen) when compared with the values obtained in the transplant recipients free of vascular complications. Abnormal blood viscosity factors may be as important as hyperlipidemia and hyperfibrinogenemia in thrombotic arterial disease in these patients. The prognostic significance of blood viscosity studies is shown in the 2 patients who had abnormal results while clinically free of manifest arterial disease, 1 of whom subsequently developed retinal arterial thrombosis (with a loss of vision) and another experienced a rejection of the graft. The correlations found in all the transplant recipients showed a significant relationship between some of the blood viscosity factors and fibringogen or serum triglyceride.