Apolipoprotein B-48 (apoB-48) is a marker of triglyceride-rich lipoprotein (TRL) remnants of intestinal origin. Chylomicron remnants are causally related to atherosclerosis. We have shown previously that fasting plasma apoB-48 may predict postprandial lipaemia. Remnant-like particle-cholesterol (RLP-C) may also reflect TRL remnants. We aimed to determine whether subjects with heterozygous familial hypercholesterolaemia (FH) had an accumulation of remnants of intestinal origin, as reflected by fasting plasma apoB-48 and RLP-C levels. The fasting plasma concentrations of apoB-48 and RLP-C were measured in 15 subjects with heterozygous FH and 15 age- and sex-matched, normolipidaemic subjects. ApoB-48 was determined using SDS-PAGE and a western blotting/enhanced chemi-luminescence technique. RLP-C was measured using an immuno-separation assay. Serum apolipoprotein B-100 (apoB-100) levels were measured using immunonephelometry; lipids were assayed enzymatically. Compared with controls, FH subjects had significantly elevated plasma concentrations of apoB-48 (29.3 median, 16.7-45.1 mg L-1 range vs. 12.8, 7.3-28.6; P < 0.001) and RLP-C (16.2, 1.5-114.3 mg dL-1 vs. 8.5, 5.0-13.5; P = 0.003), as well as serum total apoB-100 (1.9, 1.3-2.6 g L-1 vs. 1.0, 0.3-1.3; P < 0.001), LDL-cholesterol (8.1, 4.6-10.4 mmol L-1 vs. 3.5, 2.4-4.4; P < 0.001) and triglyceride (1.5, 0.6-5.6 mmol L-1 vs. 1.0, 0.4-1.8; P = 0.018). There was no significant difference in HDL cholesterol. The findings suggest that patients with heterozygous FH have elevated plasma concentrations of TRL remnants, including those of intestinal origin. This may be a consequence of decreased clearance of these particles by the LDL-receptor.