This article on the sexuality of adolescents (aged 16 to 20) presents results of a secondary analysis of Swiss studies completed on the subject between 1972 and 1992. The aim of the study was to follow up the evolution of three aspects of sexual behaviour since 1972 as a contribution to answering the following question: "What influence have the AIDS epidemic and the 'STOP-Aids' campaign had on the time of onset of sexual activity in young people?" Three aspects of sexual behaviour have been analyzed: 1. The proportion of young people who have already had sexual intercourse. This research shows that the percentage of sexually active 17-year-olds increased in the last 20 years for girls from 33% to 46% and for boys of the same age, from 23% to 51%. This increase took place before 1987. Since then, the proportion of sexually active young people of this age has not changed. 2. The number of life-time partners. In 1992, 28% of girls and 23% of boys aged 17 to 20 years had had more than 4 sexual partners. There had been no changes in these percentages since 1987. 3. The use of contraception. The use of contraception during the first sexual intercourse has become more frequent. In girls aged 16 to 20 years it rose from 47% in 1976 to 76% in 1992; in boys it increased from 50% in 1976 to 76% in 1992. For 17- to 20-year-olds, condom use with an occasional sexual partner rose from 16% in 1987 to 69% in 1992.(ABSTRACT TRUNCATED AT 250 WORDS)