Parental attitudes towards young people's sexuality in traditional (i.e. non-online media) settings have been associated with young people's sexual activities. In this study, we explored the association between key parent and youth characteristics and parental attitudes towards young people's online sexual activities. We also examined the association between young people's self-reported online sexual activities and parents' attitudes. Questionnaires were completed by parents and young people in 496 families. Parents' attitudes towards young people's offline and online sexual activities were closely related, although parents are more accepting in an offline setting. Parents' attitudes towards online sexuality are related to their sexual attitudes and their preferences with regard to the Internet. Parents' attitudes differ depending on the sex of the parent and the sex and age of the child. The link between parents' attitudes and young people's online sexual activities appears to be mediated by parental rules.