The pathogenesis of diabetic retinopathy is not clear, but it is generally agreed, that in addition to vascular components hemorheological disturbances can play a role in the impairment of microvascular flow. The aim of this study was to correlate the severity of retinopathy in diabetic patients with red blood aggregation and some parameters of aggregation. The study was carried out on 52 patients suffering from type 2 diabetes mellitus with non-proliferative retinopathy--25 men and 27 women (mean age 63.8 +/- 6.5 years) and 43 healthy--20 men and 23 women (mean age 55.5 +/- 9.6 years). The fundus appearance of patients showed non-proliferative diabetic retinopathy. Red blood cell aggregation was determined by LORCA aggregometer. Aggregation index (AI), disaggregation shear rate (gamma(p)), time to the half of the disaggregation (t1/2), time constant of fast component in syllectogram (T1), time constant of slow component (T2) and amplitude (total extent of aggregation Am) were measured. In diabetics with non-proliferative retinopathy we observed the increase of AI (66.5 +/- 7.2 vs 59.4 +/- 5.8; p < 0.001), decrease of Am (19.8 +/- 3.3 vs 21.2 +/- 2.7; p < 0.05), decrease of T1 (2.4 +/- 0.5 vs 2.75 +/- 0.42; p < 0.005), increase of T2 (27.6 +/- 4.7 vs 25.75 +/- 4.11; p < 0.05), decrease of t1/2 (1.98 +/- 0.72 vs 2.7 +/- 0.77; p < 0.001) and increase of gamma(p) (215.6 +/- 23.3 vs 98.1 +/- 8.7; p < 0.001). The parameters of erythrocytes aggregation were correlated with intensification of diabetic retinopathy symptoms. Our results show the importance of red blood cell aggregation parameters in the development of diabetic microangiopathy.