A fourth case is described in which phytosterolaemia, earlier diagnosed as familial hypercholesterolaemia, was associated with normocholesterolaemia, hypersplenism and premature atherosclerotic arterial disease requiring a three-vessel coronary bypass at the age of 29 years. During a follow-up of 5 years 22-26% and 27-30% of serum and bile sterols were plant sterols, respectively. In addition to campesterol and beta-sitosterol, stigmasterol and a fourth major plant sterol, tentatively identified as avenasterol, were found in bile, and in free and esterified forms in all serum lipoproteins. Analysis of faecal steroids and measurement of biliary lipid secretion indicated that in addition to enhanced absorption of plant sterols their decreased biliary secretion contributed to the development of phytosterolaemia. Impaired biliary cholesterol secretion was compensated for by a markedly reduced cholesterol but normal bile acid synthesis and resulted in bile undersaturated with respect to cholesterol, in a reduced intestinal cholesterol pool and in a very low faecal excretion of cholesterol as neutral sterols. Cholestyramine brought about a modest increase in cholesterol elimination as bile acids, increased cholesterol synthesis as evidenced by the sterol balance value and the increased cholesterol precursors squalene and methyl sterols in plasma and bile, and reduced the plasma cholesterol by 21% and plant sterols by 16%, but had no effect on the biliary composition of main sterols.