BACKGROUND Smoking is an important health risk in general, and responsible for diseases with significant mortality and morbidity. Smoking habits start early and adolescence is a notorious time for starting smoking. OBJECTIVE To assess knowledge on smoking and smoking habits in a population of adolescents in four Porto schools, using a confidential self administered questionnaire. Collected data were evaluated using the SPSS 1.2 statistics program (2004 version). RESULTS A total of 1,770 students aged 11 - 21 (median 15.1 years), mainly female, (58%), answered. Most students (n=952, 54.6%) were unaware of signs or warnings against smoking in their schools. The great majority (n=1639, 92.7%) considered themselves well informed on the harmful effects of smoking, but only 6.7% could list three or more tobacco-associated health consequences, however. Parents and friends were seen as privileged sources of information. Among these students, 194 (11.1%) were smokers and the average started to smoke at the age of 15. The majority of these (n=111, 57.2%) had parents who smoked and 96.4% had friends who smoked, versus 83.1% of non-smokers, a statistically significant difference (p < 0.001). Pocket money was the means of acquiring cigarettes in 34.8%. Most (60.8%) considered themselves able to stop smoking at any time, while 11.4% of the smokers smoked more than one pack a day and 9.8% smoked the first cigarette within 5 minutes of waking, however. CONCLUSIONS The percentage of smokers in this group of teenagers was considerable and indicators of nicotine dependence were found. Knowledge of the risks of smoking was poor and information on smoking given by schools had an apparently low and variable impact. Parents' and friends' behaviour may have a weighty impact on the decision to start smoking.