Serum lipids are transported by specific proteins, the apolipoproteins. These apolipoproteins serve various functions, e.g. the activation of enzymes which are important for lipoprotein metabolism or the binding to receptors on celJ membranes. Apolipoproteins, therefore, are not only ofinterest for diagnostic purpose, but also to get insight into the pathogenetic mechanisms, which lead to lipid disorders. The diagnostic relevance is illustrated with studies on two groups ofpatients with pectanginous complaints: one group consisting of patients with significant stenoses of the coronary arteries, and one group without stenoses. The combined determination ofapo-l and cholesterol proved to be the best discriminant between the two groups. The pathogenetic relevance of apolipoprotein measurement is demonstrated with 6 patients suffering from chylomicronemia. Four out of these displayed an apo C-ll deficiency which was the cause of the absence of lipoprotein lipase activity, one patient showed a deficiency of the enzyme itself, while one patiem had a chylomicronemia of unknown origin, since both apo C-ll and lipoprotein lipase activity were within normal values.-