In order to ascertain whether the increased aggregability observed in the red blood cells of diabetic patients is induced exclusively by plasma factors or is also influenced by membrane lipids, we examined the phospholipids of the erythrocyte membrane, the plasma fibrinogen concentration and erythrocyte aggregation in 86 insulin and non insulin-dependent diabetic patients. The data obtained show that the erythrocyte aggregability of the diabetic patients is higher than that of the control group (10.0+/-2.4 vs. 7.8+/-1.6%). This increased aggregability correlates not only with a higher fibrinogen concentration but also with changes observed in the membrane phospholipids. The percentage of sphingomyelin (SP) in the patients is higher than in the controls (22.6+/-6.8 vs. 18.4+/-5.4%) and that of phosphatidylserine (PS) is lower (9.5+/-6.1 vs. 12.1+/-5.1%). No differences in the percentages of the other two phospholipids identified (phosphatidylcholine, PC, and phosphatidylethanolamine, PE) were observed. The increase in the saturated nature of the phospholipids of the erythrocyte membrane, which can be measured by the (SP + PC)/(PE + PS) ratio, is statistically related (r = 0.39; p < 0.05) to the increased red blood cell aggregability observed in these patients.