Lipoprotein(a) [Lp(a)] is thought to be atherogenic in part because of the similarity of its apolipoprotein (apo) (a) component to plasminogen. Although considerable data in vitro suggest that Lp(a) could be thrombogenic by competitively inhibiting plasminogen activation, not all reports agree. To date, no human studies strongly support this hypothesis. Individual variations in the apo (a) sequence may affect its ability to bind fibrin and may account for some of the variable results. In addition, other pathologic activities of Lp(a) have recently been suggested.