The hyperlipidemia of pregnancy consists primarily of an increase in triglyceride with lesser rises in cholesterol and phospholipid. As a further characterization, we have analyzed all lipids in the major lipoprotein subfractions in fasting pregnant and non-pregnant women. An elevated triglyceride in the major lipoprotein fractions in pregnancy is confirmed. The triglyceride rises in VLDL and IDL (density 1.006-1.019 lipoprotein) are associated with proportional rises in cholesterol and phospholipid. The result is a 3-4-fold increase of compositionally unchanged lipoprotein lipid. Contrasting changes are seen in LDL, density 1.019-1.063 lipoprotein, and HDL. In these fractions, triglyceride rises more than cholesterol and phospholipid. As a result, an increase in triglyceride on a percentage basis tends to reduce the contribution of the other two lipids. Nonetheless, on an absolute basis HDL cholesterol is not significantly reduced. The proportional increases in all lipids of VLDL and IDL fractions are consistent with increased VLDL production in pregnancy as suggested by data from animal systems. However, alterations in removal are not rules out. Maintenance of the HDL cholesterol level distinguishes pregnancy from other endogenous hypertriglyceridemias where HDL cholesterol is reduced. One may speculate that these physiological adaptations in material lipid transport can serve the increased energy needs of the mother, supply steroid hormone precursors for the placenta, and provide cholesterol and essential fatty acids for the fetus.