Aims: To explore adolescents' perceptions of smoking in cars and awareness of the potential harms associated with such exposure. Methods: Data were drawn from a 2011 cross-sectional study with 11-16 year olds (n = 1373) in the UK, who were asked how often they are exposed to smoking in cars, their support for banning smoking in cars and their perceptions of the harms associated with exposure, both to themselves and non-smokers in general. Findings: Almost half (46%) reported being exposed to smoking in cars, with 17% reporting travelling more than once a week with someone smoking. 85% supported a ban on smoking in cars carrying children under 16 years. 74% reported that they disliked being exposed to smoking in cars and 91% perceived exposure to be harmful. Logistic regression found that dislike of exposure, perceived harm and support for bans varied depending on smoking status, exposure to smoking in cars and their demographic profile. Conclusions: Many UK adolescents report exposure to smoking in cars. Adolescents' opposition to such exposure, the associated health risks and their support for car smoking bans highlights the need for further consideration to be given to legislative options to protect children and others in private cars.