Exposure to secondhand smoke (SHS) is a significant threat to public health, and represents a danger for both the development and health status of children and adolescents. Taking the above into account, our aim was to quantify Greek adolescents' exposure to SHS using serum cotinine levels. During 2006, 341 adolescents aged 13-17 were randomly selected from high schools in Heraklion and agreed to participate as part of the European Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) study. Blood samples were drawn from a random sample of 106 adolescents, while serum cotinine/nicotine concentrations were measured by Gas Chromatography-Mass Spectrometry (GC-MS). The mean levels of serum cotinine and nicotine were calculated at 1.60 +/- 2.18 ng/mL and 4.48 +/- 4.00 ng/mL, respectively, while 97.7% of the non-smoker adolescents were found to have measureable levels of serum cotinine indicating exposure to SHS. The analysis revealed that their paternal (p = .001) and maternal smoking habits (p = .018) as also the existence of a younger brother or sister (p = .008) were the main modifiers of SHS exposure during adolescence. Conclusively, almost all of the measured Greek adolescents were exposed to SHS, even when their parents were non-smokers. This finding indicates the need for both community and school-based educational programmes as also the implementation of a comprehensive ban on smoking in public places.