Despite the current preventive strategies sexually transmitted infections (STIs) are rising. This thesis presents a model with the aim of improving the prevention of STIs. Chlamydia being the most common STI in Sweden serves as a model infection. The presented model is based on five performed studies. First we evaluated the feasibility of taking a urine sample at home and sending it with mail for Chlamydia analysis. Postal screening was thereafter analyzed for cost effectiveness estimating the cost per prevented sequel and the prevalence threshold. A population based study of all the 22-year old men in Umeå tested the feasibility of the Internet as a tool in facilitating Chlamydia testing and also attempted to engage men who often have low participation in Chlamydia screening programs. A questionnaire to all 18-year old youths in four northern Swedish cities was analysed concerning condom use. Finally, ordering home a Chlamydia test from the Internet was tested as method for self-selective screening. The studies showed that it was both feasible and acceptable by the population to send in urine samples with mail for Chlamydia analysis. Large-scale postal screening would be cost-effective in a female population if the C. trachomatis prevalence exceeds 5% and cost effective in a male population if the C. trachomatis prevalence exceeds 12.3%. Using the Internet as a tool in Chlamydia testing proved feasible and almost 38% of all the 22-year old men in Umeå participated. Condom usage being the most important STI preventive measure during a sexual intercourse proved to be dependent on more male factors than female factors, where the use of oral contraceptives was the strongest factor associated with non-condom use. Ordering a test on the Internet proved to be a feasible self-selective screening method. Condom use should continuously be promoted to sexually active people especially if oral contraception is used. Success and cost effectiveness in controlling STI transmission is likely to depend on achieving consistent and regular coverage of testing and partner notification among both women and men. The suggested model uses the Internet as a tool in communicating preventive STI information and allowing a self-selective screening, which may be used as an adjunct to regular recruitment in striving for improving the STI prevention. Key words: STI, prevention, Chlamydia, postal screening, cost effective, condom, Internet, self-selective screening.