Abstract Objectives Lipoprotein(a) [Lp(a)] can be oxidized by copper in vitro in a way comparable to low-density lipoprotein (LDL). We sought to determine whether the susceptibility of Lp(a) to oxidation is correlated with the susceptibility of autologous heterogeneous LDL, with apolipoprotein(a) [apo(a)] molecular size, or with both factors. Design and methods We examined shifts in electrophoretic mobility of Lp(a) and LDL caused by copper oxidation in plasma samples from 81 healthy men. The effect of copper oxidation on different-sized apo(a) was also evaluated. Results There was a close correlation between the relative electrophoretic mobilities of oxidized Lp(a) and oxidized LDL in subjects, especially with small-sized apo(a) ( n = 25, r = 0.72, p < 0.0001). Oxidative processes in Lp(a) resulted in the degradation of large-, but not small-sized apo(a). Conclusions The susceptibility of Lp(a) to oxidation is correlated with that of autologous LDL. Large-sized apo(a) may be involved in the Lp(a) oxidation.