The present study was designed to study RBC aggregability in type 1 and type 2 DM by a new method based on the dielectric properties of disperse systems. This dielectric method has a significantly higher sensitivity to detect enhanced RBC aggregation in DM than other methods. Aggregability is increased in type 1 DM and even more markedly in type 2 diabetic patients. The enhanced RBC aggregation in type 1 diabetes was significantly correlated with the levels of HbA(1C), cholesterol and triglycerides. However, no correlation between metabolic control and RBC aggregability was found in type 2 DM. The in vitro addition of non-toxic, low molecular weight dextran improves the high RBC aggregation in diabetes type 2. In the future, low molecular weight dextran may be used in DM patients clinically to lower the risk for vascular complications, after the problem of filtration is solved.