To study the possible importance of the low density lipoprotein (LDL) receptor in regulating the degree of postprandial lipemia, a cholesterol-rich fat emulsion was infused into the duodenum of subjects who were divided into four groups based on the expected variation in the expression of the LDL receptor: young men (n = 11), elderly men (n = 7), male patients on estrogen therapy (n = 5), and patients with familial hypercholesterolemia (n = 9). In familial hypercholesterolemia, fasting plasma levels of lipoproteins of d less than 1.006 g/ml, intermediate density lipoproteins, and LDLs were increased. During the fat infusion, the cholesterol and triglyceride contents in the d less than 1.006 g/ml fraction increased to a similar extent in all groups, whereas a concomitant reduction of LDL cholesterol levels was observed. The degree of the decrease in LDL cholesterol was positively correlated with the observed increase in triglycerides in the d less than 1.006 g/ml fraction. There were no signs of accumulation of intermediate density lipoproteins during infusion in any of the groups studied. The results indicate that the capacity for clearance of chylomicrons and chylomicron remnants is not affected by variation in LDL receptor expression.