Erythrocyte aggregation (EA) was determined in a Myrenne aggregometer at stasis (EAMo) and low shear (EAM1) in 102 patients suffering from primary hyperlipoproteinemia (PHLP)-46 with familial hypercholesterolemia (FH); 28 with familial combined hyperlipemia (FCHL); 28 with primary hypertriglyceridemia (PHTG)-and in a control group (CG) of healthy matched subjects. EA was also determined in FH after the autologous plasma had been replaced by a control plasma. The following parameters were also measured: fibrinogen (Fbg), plasmatic lipids, apolipoproteins, glucose, HbA1 c and membrane erythrocyte lipids: cholesterol (C) and phospholipids (PL). An increase in both EAMo and EAM1 was observed in all the studied groups of patients. When erythrocytes of FH were resuspended in control plasma, EA normalized, but only in 75% of them. Fbg was elevated only in FH and FCHL. Membrane C was increased mainly in FH and FCHL. EA correlates with both Fbg and apolipoproteins. In FH, EA also correlates with membrane C/PL. In addition, a high significant correlation exists between EA and HbA1 c in FCHL. The results obtained suggest that not only Fbg and apolipoproteins but also possible changes in erythrocyte membrane could encourage EA in PHLP.