Platelet aggregation induced by threshold concentrations of ADP, adrenaline, collagen and Thrombofax® was evaluated in 25 type IIA hypercholesterolemic patients in comparison with 15 control subjects. Malondialdehyde (MDA) formation induced by collagen and thrombin was also studied in a subgroup of 8 patients. In the patient group, irreversible platelet aggregation was induced by significantly lower concentrations of both adrenaline (p < 0.05) and Thrombofax (p < 0.01). MDA formation induced by both aggregating agents was markedly increased (p < 0.01) in type IIA hypercholesterolemic patients