In the United States and Canada, cars were found to be a major source of harmful secondhand smoke (SHS) exposure among youth. Little is known about the magnitude of this public health problem in European countries. We study SHS exposure in vehicles among adolescents across seven cities of the European Union (EU), with a particular focus on socioeconomic characteristics and smoking in adolescents' social environment.Self-reported survey data on SHS exposure in cars during the past seven days was obtained from the 2016/17 cross-sectional SILNE-R study for 14- to 17 year old adolescents in seven EU cities (N = 10,481). We applied two multivariable logistic regression models with sociodemographic characteristics and mediating smoking-related factors.SHS exposure in cars varied widely across the seven EU cities: 6% in Tampere (Finland), 12% in Dublin (Ireland), 15% in Amersfoort (the Netherlands), 19% in Hanover (Germany), 23% in Coimbra (Portugal), 36% in Namur (Belgium) and 43% in Latina (Italy). Low paternal (OR 1.65, CI95% 1.38-1.98) and maternal (OR 1.40, CI95% 1.16-1.68) educational levels and parental migration (OR 1.37, CI95% 1.14-1.64) backgrounds were correlated with SHS exposure in cars. Other correlates were one's own or peer smoking and environmental family factors, such as having at least one parental smoker (OR 4.04, CI95% 3.49-4.68) and partial smoking bans at home.In most of these seven cities, a considerable proportion of youth riding in cars, particularly those from disadvantaged and smoking-permissive backgrounds, is exposed to SHS in cars.