A reduction in total plasma cholesterol concentration has been reported in insulin-dependent diabetic (IDDM) pregnant women in early gestation. To determine if this reduction extends throughout gestation and which lipoprotein fractions may be responsible, we measured plasma triglyceride, cholesterol (C), high density lipoprotein cholesterol (HDL-C) and HDL2 and HDL3-C subfractions between 6 and 36 weeks' gestation in normal and IDDM women. Total plasma C was significantly lower in IDDM pregnant subjects between 20 and 36 weeks' gestation as compared to nondiabetic controls, while plasma triglyceride concentrations were not significantly different in this interval. Very low and low density lipoprotein (VLDL, LDL) C concentrations were not statistically significantly different from controls at any of the times studied, while HDL-C was lower throughout diabetic pregnancy as compared to controls, significantly so between 20 and 36 weeks' gestation. The lower HDL-C in IDDM women was associated with a significantly lower HDL3-C level. Plasma apoprotein A-I and A-II concentrations, markers of the HDL2 and HDL3 subclasses, respectively, were measured to corroborate the HDL subfraction changes. Apo A-I and A-II increased significantly between 12 and 28 weeks' gestation in control but not in diabetic pregnant subjects, consistent with a higher HDL3 in normal than in diabetic pregnant subjects. It appears that plasma triglyceride, VLDL and LDL-C, and HDL2-C concentrations are similar in IDDM and normal pregnancy, while total-C, HDL-C and HDL3-C and its associated apoproteins are lower than in normal subjects in late gestation. The mechanism of these changes and their significance for fetal growth and development deserve further study.