Fluctuations in estrogen and progesterone during the menstrual cycle can cause changes in body systems other than the reproductive system. We conducted several studies to determine a possible correlation between phases of the menstrual cycle and specific skin properties. Healthy Caucasian women (ages 21-48), who had a typical 26-29 day menstrual cycle, participated in the studies. Measurements of skin barrier strength, dryness, response to lactic acid stinging, skin surface lipids, and microflora were obtained every week for two to three months. Ultraviolet B susceptibility in terms of minimal erythemal dose was also studied. The skin barrier was the weakest between days 22 and 26 of the cycle. Elevated neuronal response (lactic acid sting) was not observed to vary much with the cycle. Skin was driest between day 1 and day 6, while skin surface lipid secretion appeared to be highest on days 16-20 of the hormonal cycle. The highest microbial count was around days 16-22, and there was a high UV-B susceptibility between days 20 and 28 of the menstrual cycle.