BACKGROUND: The onset of sexual activity represents an important development stage with positive aspects, such as love, discovery, intimate relationships, plans for the future, but also with fears of pregnancy, of sexually transmitted diseases and of AIDS. OBJECTIVE: To study perceptions, attitudes, and behaviour related to sexual life, AIDS and contraception; to explore the onset of first sexual relationships and the process of choice of a contraceptive method by the adolescents, in order to improve prevention programmes for young people. METHOD: Analysis of data from a Swiss national survey on adolescent sexuality using a computerised self-administered questionnaire, involving 2075 girls and 2208 boys between the ages of 16 and 20. The use of computers helps improve confidentiality, response rates and acceptability since survey questions are limited to the subjects' sexual experience only. RESULTS: The young people's responses emphasised the importance of emotion in sexual relationships, girls choosing intimacy and fidelity while boys attached more importance to physical pleasure. Three out of four respondents have had a sexual experience and one out of two have had penetrative sexual intercourse. The percentages of condom or oral contraception use are high: at first sexual intercourse, 86.5% used one or the other, while 7.4% did not declare any contraceptive method. The percentages are lower when age at first intercourse is below 15 years, when a girl had an older partner (age difference 7 years and more) and when the 1st relationship is a casual one. During their first stable relationship 41.1% of girls and 30.9% of boys say they have changed their contraceptive method from condom to contraceptive pill, 2.4% of girls and 2.9% of boys say they have given up any form of contraception. Among girls, condom use at first sexual intercourse with a new partner decreases in favour of oral contraception between first and last steady relationships (75.6 vs 58.0%, p < 0.05), the decrease being insignificant between the first and last casual relationships (73.5 and 62.2%, n.s.). Among boys the rates of condom use are equal at first intercourse with the first and last partner (steady relationship: 74.1 and 77.2%; causal relationship: 78.3 and 76.2% respectively). CONCLUSION: The use of condoms is high among Swiss adolescents, particularly at first sexual intercourse. By integrating the prevention of sexually transmitted diseases and of unwanted pregnancies, preventive programmes would address adolescents' needs more effectively.