Lipoprotein(a) [Lp(a)], a lipoprotein variant, was relegated for almost 25 years to the study of a few specialists. During the past 3 to 4 years, however, there has been a tremendous upsurge of interest in Lp(a), primarily because of multidisciplinary efforts in structural and molecular biology. Findings emerging from these efforts include the following: Lp(a) represents a cholesteryl-ester, low-density-lipoprotein (LDL)-like particle with apolipoprotein (apo) B-100 linked to apo(a); apo(a) is a glycoprotein coded by a single gene locus on the long arm of chromosome 6, which has several alleles, accounting for its remarkable size polymorphism (300 to 800 kD); apo(a) size polymorphism relates to plasma levels and density distribution of Lp(a); apo(a) is strikingly similar to plasminogen; and in vitro, Lp(a), in appropriate levels, competes for some physiologic functions of plasminogen in the coagulation and fibrinolytic cascade and may thus be thrombogenic. The LDL-like properties of Lp(a) may also confer atherogenic potential, but the mechanisms underlying this atherogenicity remain to be defined. In epidemiologic studies, high plasma Lp(a) levels have been associated with an increased incidence of atherosclerotic cardiovascular disease, especially in patients less than 60 years of age. Moreover, Lp(a) has been found as an intact particle in the arterial intima, particularly in association with atherosclerotic plaque. This finding suggests that Lp(a) can transverse the endothelium, possibly by a non-receptor-mediated process, and, at the intimal level, acquire thrombogenic and atherogenic potentials. Current information justifies the need to determine plasma Lp(a) levels in patients with a history of atherosclerotic cardiovascular disease. Unfortunately, the available techniques need to be standardized. Apolipoprotein(a) exists in isoforms of different sizes, and the importance of determining apo(a) phenotypes in clinical practice remains to be established.