The effect of endogenous sex hormone fluctuations on plasma lipoprotein levels was studied in 15 young women. Plasma samples were taken during the follicular, ovulatory, and luteal phases of the menstrual cycle while the women were consuming a defined diet, similar to the average American diet. No significant effect of menstrual cycle phase on plasma total cholesterol or cholesterol lipoprotein subfraction levels was found; however, a 34% increase in total plasma triglycerides associated with an increased very low density lipoprotein triglyceride fraction was found during the ovulatory phase. These data indicate that menstrual cycle phase can have a significant effect on plasma triglyceride levels, but not on plasma cholesterol or lipoprotein fractions.