The role of estrogens in regulating the T-cell immune response and in changing the cytokine profile in biological fluids is well known. The authors monitored the activity of cytokines and interferons in the vaginal swabs of clinically healthy and herpes simplex virus (HSV)-infected women in different phases of the menstrual cycle. Differences were found in cytokine concentrations in the ovulatory and progesterone phases of the cycle. Thus, the HSV-infected women had impaired cytokine balance towards the enhanced activity of proinflammatory interleukins in the ovulatory phase of the cycle and that of anti-inflammatory interleukins in its progesterone phase. This tends to reduce a protective T-cell immune response in the virus-infected women and to maintain chronic virus infection.