The study was designed to determine eventual correlation in diabetes mellitus between reduced erythrocyte deformability and red blood cells (RBC) cholesterol content. Significantly higher RBC cholesterol amount was found in 11 diabetics (IDDM) than in 13 controls of similar age (45 +/- 10 yrs) (1.30 mg ml-1 packed cells -1 +/- 0.15 SD vs 1.19 +/- 0.10 SD, p less than 0.05). Blood filterability values have been found significantly higher (p less than 0.025) in the diabetics (42.2 +/- 11.5 SD ml sec. -1) than in controls (33.5 +/- 6.0). Flow time of RBC filterability, measured in erythrocyte saline buffered suspension (40% Hct) in order to eliminate interferences from plasmatic factors, showed a closely correlation with RBC cholesterol content (r = 0.86 p less than 0.01). A significant inverse correlation between RBC cholesterol content and serum ApoA1-HDL levels was also observed (r = -0.58, p less than 0.05). Moreover ApoA1-HDL serum values were significantly (P less than 0.01) lower in diabetics (111.8 +/- 10.1 mg/dl) than in controls (126 +/- 6.6 mg/dl). Our data underline the possible involvement of augmented erythrocyte cholesterol in impairing the microhemologic competence of RBC in diabetes mellitus. A possible relation with lower ApoA1-HDL and lecithin-cholesterol-acyl-transferase (LCAT) is examined.