A plant sterol, sitosterol, was quantitated in very low density lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins (HDL) and related to faecal steroids and cholesterol absorption in heterozygous familial hypercholesterolaemia patients with (n = 7) and without ileal bypass (n = 6). The latter had resulted in severe bile acid malabsorption but fractional cholesterol absorption was within low control limits. Serum total and LDL cholesterol and apoprotein B levels were reduced, whereas HDL cholesterol, apoprotein A-I, VLDL and HDL sitosterol concentrations were increased by the ileal exclusion, and the increase in LDL and serum total sitosterol levels was insignificant. In terms of mmol/mol of cholesterol or apoprotein B, however, the LDL and total sitosterol contents were higher in the subjects who had undergone operation. For an unknown reason the sitosterol content increased gradually within the lipoprotein particles from the lighter to the heavier lipoproteins, and the enrichment was similar in the two groups. Dietary sitosterol intake, indicated by faecal sitosterol excretion, was similar in the two groups. The contents of serum total and LDL sitosterol were positively correlated with the dietary sitosterol intake in both groups, and with the fractional cholesterol absorption only in the group not subject to operation. These associations were less consistent for sitosterol contents in other lipoproteins. We conclude that normally the serum sitosterol content reflects cholesterol absorption efficiency even in patients with familial hypercholesterolaemia, provided the dietary sitosterol intake is quite constant. In addition, for unknown reasons ileal exclusion leads to an increased lipoprotein sitosterol content.