The aim of this study was to explore the degree of correspondence between sexual behaviour in the real world and that on the internet, among seventeen year old Czechs. The adolescents, 237 girls and 225 boys, completed a computer-administered questionnaire which examined sexual behaviour offline and sexual activities on the Internet. Individuals who reported any offline sexual experience tended to engage in online sexual activities more often than those who did not. The use of the Internet for sexual purposes was similar for those who had experienced kissing and/or petting, it then increased, being similar for those who had experienced oral sex and/or sexual intercourse. Findings suggest that the relationship between the gradual extension of sexual experience offline and the gradual extension of using the Internet for sexual purposes is rather weak.