The aim of this study was to investigate sexual behavior, contraceptive use, risk factors as well as sources of sex information among first-year high-school students in Sweden. Secondly, to assess differences between genders and study programs as well as changes over a 40-year period. A repeated cross-sectional survey was conducted in two cities. A questionnaire comprising 77 items was used. The study population consisted of 415 students (63.4% females). The median age of sexual intercourse was 15 years. In total, 37% had had sexual intercourse, compared to 56.3% in 2009 and 45% in 1999 (p < 0.001), and the proportion of students who had their first sexual intercourse was not influenced by gender. More students in vocational programs (46.3%), compared to theoretical (33.3%), had experience of at least one sexual intercourse (p = 0.019). The same extend of contraception use at first and latest intercourse was reported, compared to previous studies. Forty-nine percent were mostly informed about sex from the internet, while in previous years, magazines, family and youth clinics were the main information sources. Comparing over time, students were in general less sexually experienced and less engaged in non-penetrative sex and physical intimacy. These findings call for a new approach, when designing sex and relationship education and health-care counseling in adolescents.