Previous studies linking heterosexual activity to women's menstrual cycle variability have failed to take into account the effects of gynecological maturity. One hundred thirty-two women, all at least seven years postmenarche and not using birth control pills, completed daily records of their cycles and their heterosexual behavior. Data from women classified as sexually celibate or as regularly sexually active (having sex at least once per week in every nonmenstruating week) replicated previous findings while controlling for gynecological maturity: Women classified as celibate had more variable cycles than women who engaged regularly in heterosexual activity. An interaction between gynecological maturity and sexual status was also found, precluding a comparison involving women who were sexually active on an irregular basis. The interaction revealed that increased gynecological maturity is associated with less variable cycles in the sexually sporadic women, but is not associated with cycle variability in either celibate or sexually regular women. Possible biological mechanisms for these findings and their implications are discussed.