Oxandrolone, an anabolic androgenic steroid, has been shown repeatedly to lower plasma triglycerides in hypertriglyceridemic patients. This study was performed to determine which of seven subfractions of triglyceride-rich lipoproteins are affected by the action of oxandrolone with respect to both their plasma levels and composition. Concurrently, we have determined the levels and composition of HDL subfractions and the plasma levels of the major HDL apoprotein, apoA-I. Oxandrolone was administered to two hypertriglyceridemic subjects, one with type III and one with type V hyperlipoproteinemia until plasma triglycerides were below the target level of 270 mg/dl. Two months and two weeks were required for the type III and type V patients, respectively. In both subjects, the treatment caused a reduction in the plasma levels of all seven subclasses of triglyceride-rich lipoproteins without altering their overall composition. LDL were at least temporarily increased. The reduction of VLDL subfractions caused by oxandrolone was accompanied by a progressive and consistent effect on HDL subfractions in both hypertriglyceridemic subjects; in the type III patient, oxandrolone reduced HDL2 from low pretreatment levels further until they became undetectable. The type V subject had no detectable HDL2 levels prior to treatment. In both subjects, oxandrolone lowered the levels of HDL3. This lowering effect was caused by a preferential reduction of the less dense, major HDL3 subfraction, i.e. HDL3L, causing the denser, smaller HDL3 subfraction, HDL3D, to become the predominant HDL class. The lowering of HDL levels was reflected by a decrease in the plasma levels of the major HDL apoprotein, apoA-I. This first report on the simultaneous reduction of VLDL and the larger, less dense HDL subclasses suggest that oxandrolone lowers plasma triglycerides by a mechanism other than increased lipolysis.