A questionnaire-based investigation was performed to elucidate factors with possible influence on smoking status in 557 14-17-year-old boarding school pupils. A representative sample of boarding school pupils in Denmark participated. Before entering boarding school an increased risk of being a smoker was present if a majority of one's schoolmates were smokers (odds ratio 5.9; P < 0.0001); if parents were not living together (odds ratio 2.1; P < 0.005); if older brother or sister was a smoker (odds ratio 3.1; P < 0.0001); and if both parents smoked (odds ratio 1.9; P < 0.01). After changing to boarding school there was an increased risk of starting to smoke if a majority of the schoolmates were smokers (odds ratio 4.6; P < 0.0001). Believing that smoking causes damage to the smokers' own health decreased the risk (odds ratio 2.9; P < 0.01). More efficient smoking prevention programmes should be created by aiming particularly at the special problems encountered in certain age groups, and possibly through a better understanding of the influence that psycho-social factors may have on youngsters' risk of becoming smokers.