Lipoprotein(a) [Lp(a)] concentrations are known to be stable under various dietary and drug regimens. Little is known about the influence of hyperlipoproteinemia on Lp(a) levels. Therefore, we investigated Lp(a) concentrations and apolipoprotein(a) [apo(a)] polymorphism in 147 patients with hypertriglyceridemia and in 93 patients with hypercholesterolemia and compared them with 404 subjects without hyperlipoproteinemia (controls). Despite a similar apo(a) isoform and phenotype distribution, Lp(a) concentrations differed significantly (P < .0001) between the three groups. The median Lp(a) level in control subjects was 17 mg/dL (mean, 38 mg/dL), compared with 38 mg/dL (mean, 56 mg/dL) in patients with hypercholesterolemia and 9 mg/dL (mean, 21 mg/dL) in those with hypertriglyceridemia. These differences persisted after exclusion of 61 subjects with coronary heart disease. The inverse correlation between the molecular weight of the apo(a) isoforms and the Lp(a) concentration was preserved within each group (P < .001), but for every molecular weight range studied the level of Lp(a) was always higher in patients with hypercholesterolemia and always lower in those with hypertriglyceridemia than in controls. We conclude that hypertriglyceridemia or hypercholesterolemia have profound--but divergent--influences on the concentration of Lp(a).