Sweden was the first country to introduce online Chlamydia trachomatis testing for both men and women and this article provides information about the user characteristics and which risk groups are reached with this method. During the years 2005-2007 a questionnaire was administered with a response rate of 86% (n = 6025) (62.5% women, 37.5% men). Over 60% of respondents were aged below 26 years and single. The average years of education for men and women was 14 (range 9.0-26.0). The mean Internet usage time was 22.4 hours/week among men and 15.5 hours/week among women. Only 22% men and 34% women had tested themselves for C. trachomatis at clinics previously. Respondents reported a higher frequency of previous sexually transmitted infections (STIs) compared with other Internet users (i.e. C. trachomatis: men 19%, women 24%). Eighty percent of online test users had engaged in sex with ≥ 2 partners without a condom during the previous year. Online C. trachomatis test users are mostly young people with high sexual risk behaviour.