Acute changes in low density lipoprotein cholesterol levels may be due to both a change in the number of LDL particles/ml of plasma and an alteration in the amount of cholesterol per LDL particle. Since LDL cholesterol levels are known to alter abruptly after myocardial infarction, the composition of LDL was determined in nine patients who suffered an uncomplicated transmural myocardial infarction. In six of these, LDL cholesterol levels fell whereas in three LDL cholesterol rose during the first nine days in hospital. The contents of B protein, free cholesterol, phospholipid, cholesterol ester and triglyceride in LDL were determined in the initial sample and the subsequent sample showing the greatest changes in LDL cholesterol level. The proportion of the LDL molecule contributed by B protein, free cholesterol and phospholipid did not differ significantly between the two samples. In contrast, when LDL cholesterol fell, the decrease in the proportion of cholesterol ester was disproportionately greater than in triglyceride. The opposite was observed when LDL cholesterol rose. This inverse relation between changes in LDL cholesterol ester and triglyceride could be expressed by Y = -1.02 X -0.17 (r = -0.94). These data are consistent with a pseudomicellar model of LDL in which the surface components are present in fixed amounts but the interior shell of cholesterol ester and triglyceride varies in an inverse relation depending on the absolute LDL concentration.