Alterations in blood rheological properties have been reported in diabetes mellitus. Changes in lipid composition of red blood cell (RBC) membranes resulting in an impairment of RBC deformability may play a role in the altered blood rheological pattern. The aim of this study was to investigate the lipid composition of RBC membrane in a group of patients affected by type II diabetes (age 21-45 years), selected on the basis of the absence of complications and good metabolic control, and in a group of suitable control subjects. Saturated fatty acid amounts in the different phospholipid fractions were significantly higher in diabetics than in controls (p less than 0.05), whereas polyunsaturated fatty acids were decreased (p less than 0.05). Cholesterol/phospholipid molar ratio was not altered. On the contrary, sphingomyelin/phosphatidylcholine ratio was higher in diabetics than in controls (1.10 +/- 0.08 vs 0.96 +/- 0.10, p less than 0.01) due specially to high levels of sphingomyelin. These alterations could account for the impairement of RBC deformability frequently reported in diabetes mellitus, independently of metabolic control and the presence of severe atherosclerotic lesions.