The overall objective of this study was to conduct a longitudinal assessment of the association of endogenous hormones with biomarkers of oxidative stress and antioxidant status during the menstrual cycle. There were four main objectives. The first was to study the intra-menstrual cycle variation of various measures of oxidative stress. This objective is intended to assess variation in several measures of oxidative stress during different phases of the menstrual cycle, including F2-8-isoprostanes in serum. Assessment of variation across individuals is planned. The second objective was to determine the relationship between specific reproductive hormone levels and oxidative stress during specific times in the menstrual cycle of premenopausal women. The panel of reproductive hormones in the blood that were of primary interest are oestradiol, progesterone, LH, FSH and sex hormone binding globulin (SHBG). The third objective was to examine the influence of external factors on both oxidative stress and hormone levels, and their interrelation. The study measured various biological factors that might influence oxidative stress, including serum concentration of certain antioxidant vitamin levels (retinoids, tocopherols, carotenoids and ascorbic acid). In addition, the study assessed other factors that might affect oxidative stress such as medication and supplement intake, cigarette smoking, alcohol consumption, dietary intake, physical activity and levels of stress. Lastly, the study was designed to evaluate the validity and reproducibility of the various biological markers included in the BioCycle study.