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 is less than 0.05) and Thrombofax (p is less than 0.01). MDA formation induced by both aggregating agents was markedly increased (p is less than 0.01) in type IIA hypercholesterolemic patients.