Caregiver-youth communication about sex protects youth against HIV/AIDS, and caregivers who believe that sex knowledge is important are more likely to talk to their youth about sex. However, caregivers who experience barriers to communication about sex may not talk to their youth about sex even if the caregiver believes that sex education is important. The Theory of Planned Behaviour predicts that an actor has perceived control is necessary for behavioural change. This study therefore hypothesised that caregivers' perceived control moderates the relationship between caregiver attitudes about youth sex knowledge and caregiver-youth communication about sex. Results from a sample of 99 female South African caregivers of adolescent (10-14 year old) youth supported our hypothesis, indicating that caregiver attitudes about providing youth with sex knowledge positively predict communication about sex only when caregivers have perceived control. This finding illustrates the importance of perceived control in predicting caregiver-youth communication, and therefore has implications for family-based interventions aimed at improving caregiver-youth communication about sex.