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The present study was designed to evaluate the metabolism of chylomicron and chylomicron remnants by measuring serum apolipoprotein B-48 (apoB-48) levels in 335 normolipidemic and 253 hyperlipidemic subjects using a novel ELISA system. The distribution of fasting serum apoB-48 levels in normolipidemic subjects varied widely, ranging from <1 to >24 μg/ml (mean, 5.2 ± 3.8 μg/ml; median, 3.9 μg/ml). Serum apoB-48 levels correlated with serum triglyceride (TG) concentrations (r = 0.45, P < 0.001), but not with total cholesterol levels. Serum apoB-48 levels were 7 to 18 times higher in patients with Type I, Type V, and Type III hyperlipidemia, and only slightly higher in patients with Type IIa, Type IIb, and Type IV hyperlipidemia, compared with normolipidemic subjects. The calculated apoB-48/TG ratio was elevated only in patients with dysbetalipoproteinemia (apoE2/2 phenotype). In normolipidemic subjects, oral fat loading resulted in about 2-fold increase in serum apoB-48 levels, with a peak level recorded at 3–4 h postloading, and then returned to the baseline level within 6 h. On the other hand, in patients with dysbetalipoproteinemia, serum apoB-48 levels did not change considerably.Our results indicate that serum apoB-48 is a very useful parameter for evaluating lipoprotein metabolism in exogenous pathways.