Simultaneous pancreas and kidney transplantation (PKT) is associated with a deterioration of hemorheology. We investigated the determinants of plasma and blood viscosity (hct. 35%) after PKT (n = 49), in type 1 diabetes (n = 26) and in healthy controls (n = 24). Patients after PKT were subdivided due to their graft function (intact pancreas and kidney graft, n = 26; pancreas rejected, intact kidney graft, n = 23). We examined the correlations of total serum protein, albumin, fibrinogen, alpha 2-macroglobulin, total cholesterol, LDL cholesterol, HDL cholesterol and triglycerides with plasma and blood viscosity (hct. 35%) measured at a continuous shear range of 600-0.2 s-1 with a rotational viscometer (Haake, Germany). Total protein was strongly associated with plasma viscosity in all examined groups (r > 0.5, p < 0.03), it determined blood viscosity over the whole shear range in type 1 diabetic patients, but only at high shear rates after PKT (> or = 100 s-1). The strong association of albumin and blood viscosity in type 1 diabetes and in healthy controls (shear rates > or = 10 s-1) was not found after PKT. Fibrinogen correlated with plasma and blood viscosity (> or = 25 s-1) after PKT (p < 0.03) but no in type 1 diabetic patients or healthy controls. Alpha 2-macroglobulin correlated with plasma and high shear blood viscosity after PKT only after pancreas rejection, no correlation was found after successful PKT. It also correlated with plasma and blood viscosity at low and high shear rates in type 1 diabetes. Total cholesterol and low shear blood viscosity correlated positively in successfully transplanted patients (r > 0.44), but negatively after pancreas rejection (r > -0.44). No correlation was found in type 1 diabetic patients, a positive association was found in healthy controls for plasma and low shear blood viscosity. LDL cholesterol correlated negatively (after pancreas rejection) or positively (healthy controls) with low shear blood viscosity (p < 0.03) and positively with plasma viscosity. HDL cholesterol was negatively associated with high shear blood viscosity in all groups (p < 0.05), except after successful PKT, where no association was found. It did not correlate with plasma viscosity in any group. Triglycerides did not contribute significantly to blood viscosity in the examined groups. The metabolic alterations after PKT influence plasma proteins, lipids and corpuscular elements of blood with regard to their effect on rheology.