Glucose-rich plasma is commonly observed in diabetes mellitus and in-vitro incubation of erythrocytes in glucose-rich media may produce the non-enzymatic glycosylation of erythrocyte proteins. The present study investigates the effects of an increased concentration of glucose in a suspending medium on erythrocyte rheological parameters. Erythrocytes, which were obtained from ten healthy volunteers, were washed and incubated in vitro with glucose solutions at different concentrations and incubation times. The measured hemorheological parameters included deformability and the aggregation of erythrocytes. Hemoglobin concentration in the erythrocyte suspension in autologous plasma was also measured after incubation in order to assess the hemolysis of erythrocytes. Significant hemorheological changes were observed with an increase of glucose concentration and incubating time. Both deformability and the aggregation of erythrocytes decreased in dose- and time-dependent manner. A reduction in hemoglobin concentration in the erythrocyte suspensions was also observed due to hemolysis of fragile erythrocytes during incubation. Modification in the hemorheological properties of cells, which may be associated with due to glucose-induced (auto)oxidation and glycation of erythrocyte proteins in hyperglycemia, can adversely affect the fluidity and oxygen-delivery function of the erythrocytes.