The erythrocyte (RBC) deformability and lipid composition of RBC membrane were investigated in non-insulin dependent diabetes mellitus (NIDDM) patients (n=20) and healthy controls (n=15). The RBC deformability, whole blood and plasma viscocity were significantly higher in NIDDM patients than in controls (p<0.001). No difference in the RBC count and MCV were found between NIDDM patients and controls. Total cholesterol (TC), total phospholipid (TPL) levels of the plasma were significantly high (P<0.001) and no significant differences were observed in the levels of low density-cholesterol (LDL-C), and high density-cholesterol (HDL-C) in patients with NIDDM compared with the control group. In RBC, TC levels of NIDDM patients were found to be increased (P<0.001) when compared with controls. The levels of TPL did not change in RBC between NIDDM patients and controls. In the plasma and RBC, the ratio of TC/TPL was found to be increased in NIDDM patients compared with control group. A similar trend was observed for RBC phosphatidyicholine (PC) levels, but in this case, the increase was not statistical significantly. This finding shows that RBC deformability is regulated by lipid compositions of the cell membrane in the NIDDM patients. This regulation plays an important role between diabetes and vascular diseases.