BACKGROUND Based on a large national survey on the health of adolescents, this paper focuses on the socio-demographic and lifestyle correlates of sport practice among Swiss adolescents. The SMASH2002 database includes 7428 vocational apprentices and high school students between the ages of 16 and 20 who answered a self-administered anonymous questionnaire containing 565 items targeting perceived health, health attitudes and behaviour. Weekly episodes of extracurricular sport activity were measured by a four-category scale, and the sample was dichotomised between active (>or=two episodes of sport/week) and inactive (<two episodes of sport/week) respondents. Thirty percent of female respondents and 40.2% of male respondents reported engaging in sport activity at least two to three times a week; another 9.7% of the female and 19.4% of the male respondents reported participating in least one sport activity each day (p<0.01). The percentage of active respondents was higher among students than among vocational apprentices (p<.01), and the rates of sport activity decreased more sharply over time among the apprentices than among the students (p<0.01). Most active adolescents reported having a better feeling of well-being than their inactive peers [among male students: odds ratio (OR): 3.13; 95% confidence interval (95%CI): 1.28-7.70]. The percentage of active females who reported being on a diet was high, and female apprentices exhibited higher involvement in dieting than their inactive peers (OR: 1.68; 95%CI: 1.32-2.14). Relative to the inactive male respondents, the proportion of active male respondents smoking was lower; however, a lower proportion of the latter group did not report drunkenness, and the percentage of those who reported lifetime cannabis consumption was higher among active than inactive students (females, OR:1.57; 95%CI:1.09-2.25; males, OR:1.80; 95%CI: 20-2.69). CONCLUSIONS Organised sport activities should be better tailored to the work schedules of apprentices. Practitioners should be aware of the potential for problematic behaviour in the area of dieting and substance use among a subset of sport-oriented adolescents.