Oxidative cleavage of arachidonate (C-20) and linoleate (C-18) phospholipids generates truncated C-8 or C-12 gamma-hydroxyalkenal phospholipids as well as C-5 or C-9 carboxyalkanoate phospholipids, which are abundant in atherosclerotic plaques. The gamma-hydroxyalkenals promote foam cell formation by scavenger receptor CD36-mediated endocytosis. The carboxyalkanoates are potent regulators of endothelial cell functions that may promote atherogenesis. We now report an unexpected biosynthetic interconnection; the carboxyalkanoates can be generated through oxidative cleavage of the gamma-hydroxyalkenals with the loss of three carbons. This unprecedented transformation is shown to involve Michael addition of an acylperoxy radical and fragmentation of the resulting beta-hydroxyperester.