Because smoking begins most often in adolescence it is important to define clearly, with a view to prevention, the motivation of an adolescent to smoke. The role of the social group is well known. The aim of this study was to evaluate the potentially preventative role of knowledge in the field of respiration and the effects of cigarette smoke on one hand and of involvement in activities involving breathing on the other. The group studied was made up of 1,802 pupils at state schools, randomly selected, in the city of Marseilles. These pupils filled in an anonymous questionnaire in the classroom. The overall percentage of non-responders was very small. Overall 10.5% if the children declared that they had already smoked, more often boys (13.1%) than girls (8.1%). In contrast to smoking by the father, smoking by the mother and siblings significantly influenced smoking in the child. The child was not influenced by smoking by a sibling of the same sex. The proportion of children having already smoked increased progressively in proportion to the number of smokers in the household. Using a logistical regression analysis the following were predictive of smoking: being a boy, having a best friend who smoked, and the number of smokers in the family. On the other hand a history of allergy, an understanding of the effects of the environment on the respiratory system, knowledge of the effects of cigarettes, and finally involvement in sport, playing a wind instrument or singing in a choir were not associated with a lower incidence of smoking. These results call into question the effectiveness of the standard preventative methods and of anti-smoking programmes that are based on such strategies.