Serum concentrations of apolipoprotein(a) were measured in patients with heterozygous familial hypercholesterolaemia. The levels in 47 patients were a median of 2.5 times higher than those in controls matched for age and sex (240 [range 25-1245] vs 97 [7-1040] mg/l). Among patients with familial hypercholesterolaemia apo(a) levels were higher in those with (n = 48) than in those without (n = 72) ischaemic heart disease (283 [18-1245] vs 144 [7-741] mg/l); both in univariate and multivariate analysis serum apo(a) was the most significant variable distinguishing between the groups. Despite reducing LDL cholesterol by 30%, treatment with cholestyramine or pravastatin did not reduce apo(a) levels in these patients. These findings support the concept that apo(a) concentration is a genetic trait predisposing to ischaemic heart disease and imply that it may be useful in the identification of familial hypercholesterolaemia patients at high risk of coronary disease.