OBJECTIVE To examine timing of parent-child discussions about sexual topics relative to child-reported sexual behavior. METHODS Longitudinal study of employed parents and their children, with an initial survey followed by subsequent surveys 3, 6, and 12 months later. Participants were 141 parents, along with their children (13-17 years), who were control participants in a randomized, controlled trial to evaluate a worksite-based intervention to improve parent-adolescent communication. Main outcomes were parent and child reports of discussion of up to 24 sexual topics and presexual and sexual acts (ranging from handholding to vaginal intercourse) that occurred before the first survey and in the intervals between subsequent pairs of surveys. RESULTS Sexual topics tend to group into 3 sets. The first set includes topics such as girls' bodies and menstruation and typically coincides with children's presexual stage (handholding, kissing). The second set includes topics such as birth control efficacy and refusing sex and typically coincides with the precoital stage (genital touching and oral sex). The third set typically occurs when children have initiated intercourse. Over half of children engage in genital touching before discussing birth control efficacy, resisting partner pressure for sex, sexually transmitted disease symptoms, condom use, choosing birth control, or partner condom refusal; >40% of children have intercourse before any discussion about sexually transmitted disease symptoms, condom use, choosing birth control, or partner condom refusal. CONCLUSIONS Many parents and adolescents do not talk about important sexual topics before adolescents' sexual debut. Clinicians can facilitate this communication by providing parents with information about sexual behavior of adolescents.