Vaginal biopsies were obtained in 5 healthy women every third day during the menstrual cycle. The specimens were examined with morphometric technique and the mean nuclear size was correlated to values of estradiol and progesterone and caryopycnotic index. Cyclical changes of the histological appearance of the epithelium could be demonstrated. During estrogen influence a proliferation and maturation of the vaginal mucosa was seen whereas addition of progesterone was associated with a shedding of the superficial cell layers and a steady state of the basal part of the epithelium. At ovulation the epithelium reached maximal thickness and maturity. The histological architecture at this time may resemble a human-papilloma-virus (HPV)-infected mucosa. The difficulties in distinguishing an HPV-infected epithelium from normal are discussed.