AbstractHyperapobetalipoproteinemia (HyperapoB) is a lipid disorder characterized by premature coronary disease, although the mechanisms have not been elucidated. Because abnormalities in platelet function may represent an enhanced susceptibility to coronary thrombosis, the aggregability of platelets was examined in hyperapoB subjects. Compared to controls, there were no significant differences in either platelet lipid composition or in aggregation to epinephrine, ADP, or collagen. In contrast to other dyslipidemias, platelet function does not appear to be abnormal in this well defined lipid disorder.