Erythrocytes from diabetic patients exhibit impaired viscoelastic properties when estimated by various methods. We determined erythrocyte filterability through 5-microm pores, in 51 patients with non-insulin-dependent diabetes mellitus, 18 healthy controls, 15 patients with homozygous beta-thalassemia and 15 with beta-thalassemia traits. The filtration measurements were made with a Hemorheometer, which uses the "initial flow rate" principle. To determine the Index of Rigidity (IR) of the red blood cells, we measured the passage time of white blood cell-free erythrocyte suspensions, 8% per volume, through the filter. Diabetic patients had significantly increased IR in comparison to healthy controls and to patients with beta-thalassemia trait, but not at the level found in patients with homozygous beta-thalassemia. In diabetic patients, a strong correlation between IR and the percentage of glycosylated haemoglobin was found (r=0.737, P < 0.0001), and a weaker one with serum unconjugated bilirubin (r=0.363, P=0.0097) and serum total lipids (r=0.321, P=0.0286). Patients with severe retinopathy also had significantly increased IR, in comparison to those with or without mild retinopathy. Anaemic diabetic patients, especially those with the anaemia of chronic disease, also had significantly increased IR in comparison to non-anaemic diabetics. No correlation between IR, MCV, MCH, MCHC, RDW, RBC morphology, serum LDH or the presence of erythrocyte inclusions after incubation with nitrous sodium solution was found. Our findings suggest that glycosylation of skeletal proteins probably contributes significantly to the increased membrane rigidity of diabetic erythrocytes.