Smoking and its risk factors were studied in 1990 on a representative sample of 2,514 pupils in Yvelines by auto-questionnaire. A similar enquiry had been carried out in 1983 with the same methodology. It was noted that between 1983 and 1990, there was a diminution in the prevalence of smoking from 15.4 to 7.6 per cent in the young in Yvelines. Amongst those young people of French nationality, the prevalence of smoking passed from 16 to 9 per cent in boys and from 17 to 12 per cent in girls aged between 11 and 16. This diminution of prevalence affected both brothers, sisters, and best friends whether male or female. The proportion of youth who had never smoked was significantly increased. In 1990, age and smoking habits of best male or female friend had a relationship which was independent of other factors with smoking in the two sexes. In girls, religious practice and (just within the limits of significance) the attitude of parents, and in boys the absence of one parent from the home, were also linked to smoking habits. The evolution of risk factors was discussed, particularly comparing the two studies of 1983 and 1990. All health education should take account of the phenomenon of "youth groups" as initiation occurs early, health education should be made early, at primary school level.