29 cadaveric renal transplant recipients were assessed clinically for evidence of occlusive arterial disease prior to undergoing blood viscosity studies. Nineteen patients had manifest arterial disease (myocardial infarction, cerebral thrombosis, angina, intermittent claudication, absent peripheral pulses), while ten were free from vascular complications. Patients with arterial disease showed significant elevations of plasma viscosity (p < 0.005), aggregation of red cells measured both at 37 and 20°C (p < 0.05), fibrinogen (p < 0.005), serum triglyceride (p < 0.01), serum cholesterol (p < 0.01), erythrocyte sedimentation rate (p < 0.02), and a significant reduction in the albumin/fibrinogen ratio (p < 0.005) when compared with those free of disease. Two patients with no apparent vascular disease when investigated were found to have distinctly abnormal blood viscosity factors, and one subsequently developed retinal arterial thrombosis while the other suffered serious damage of the graft within 3 months of viscosity study. When all patients were considered together, significant correlations were found between viscosity of artificial thrombi or aggregation of red cells and fibrinogen level (both p < 0.05), and serum triglyceride level (both p < 0.05); and between rigidity of red cells and the parathyroid hormone level (p < 0.01).