To determine if postpartum lactation alters plasma lipoprotein lipid and apoprotein concentrations and composition, we studied 56 overnight fasting lactating and 16 nonlactating women approximately 6 weeks postpartum. Postpartum results are presented as absolute concentrations and as the difference from antepartum values determined at 36 weeks gestation. Antepartum lipoprotein lipid and apoprotein concentrations were generally not different in the 2 groups, with the single exception of whole plasma and low density lipoprotein (LDL) apoprotein (apo) B (probably a chance difference). When expressed as the antepartum and postpartum difference, the lactating and nonlactating groups were indistinguishable in very low density lipoprotein (VLDL) and LDL triglyceride, cholesterol, phospholipid, and apo B concentrations. However, lactating women had higher high density lipoprotein (HDL) cholesterol, phospholipid, apo A-I, and apo A-II concentrations than nonlactating women when results were expressed as differences from antepartum values or as absolute values. HDL triglyceride concentrations were not significantly different between lactating and nonlactating women by either analysis. There was no significant effect of lactation on VLDL or LDL composition, but there was a significant increase in the percent cholesterol content in HDL. We hypothesize that the increase in HDL constituents in lactation is generated in part by increased catabolism of triglyceride-rich lipoproteins by the lactating breast.