A 62 year old woman with primary biliary cirrhosis was found to have a plasma sodium concentration of 115 mmol/l. Follow up showed this to be a "pseudohyponatraemia" due to a massively raised serum cholesterol concentration of 78 mmol/l. Electrophoresis of serum lipoproteins and of the lipoproteins and apolipoproteins in fractions isolated on density-gradient ultracentrifugation showed that the major portion of the serum cholesterol was being transported with lipoprotein-X. Low density lipoprotein-cholesterol concentration was also raised. Lipoprotein-X contained, in addition to albumin and apolipoprotein C, apolipoprotein E. This case is of interest because of the degree of hypercholesterolaemia, its association with pseudohyponatraemia, and the unequivocal demonstration of apolipoprotein E associated with lipoprotein-X.