The associations between tobacco smoking and demographic, socioeconomic, environmental, and behavioral factors among adolescents were investigated by administering an anonymous questionnaire to 5,221 ninth (aged 14-15 years) and 4,154 thirteenth grade (aged 18-19 years) students in 10 Italian towns. Using logistic regression analysis, both current smoking and experimental smoking were statistically associated with sibling, best friend, and partner smoking, alcohol drinking, and frequency of drunkenness in both grades and sexes. Attitude of parents toward children's smoking was also found to be associated with the probability of being a current smoker, especially among ninth graders. Parental smoking was associated with current smoking among females only. Socioeconomic factors and level of knowledge of the health consequences of smoking were not associated with either experimental or regular smoking. In conclusion, this survey suggests that social environment plays a fundamental role in both first experimentation with smoking and the process of becoming a regular smoker among adolescents. Students with one or more siblings who smoke, and especially those with best friends and a partner who smoke, were much more likely to have tried smoking and to be current smokers than students without smokers in their environment.