An individual's sex influences plasma human immunodeficiency virus type 1 (HIV-1) RNA level and rate of CD4 cell decline, but the mechanism for this effect is currently unknown. To determine the effect of the ovulatory cycle on HIV-1 RNA level and lymphocyte subsets in HIV-infected women, blood specimens were obtained weekly from 14 women infected with HIV. Participants reported regular menses and were not using hormonal medications or narcotics. The occurrence of ovulation was verified by use of endocrine criteria. Ovulation occurred in 10 of the 14 women. Among women who ovulated, median HIV-1 RNA level fell by a median of 0.16 log10 from the early follicular phase to the midluteal phase (P=.03, Wilcoxon signed-rank test). When women who did not ovulate were included in the analysis, no significant fluctuation in plasma HIV RNA level was identified. Thus, the ovulatory cycle influenced circulating HIV-1 RNA levels, a finding that is plausible because of the known effect of sex hormones on lymphocyte function and cytokine production.