['Contact tracing is proved to be a good way to prevent asymptomatic sexually transmitted infections, such as Chlamydia trachomatis, from spreading. According to the Swedish law a physician has to report all cases of genital C. trachomatis to the County Medical Officer of Health and to the National institute for Infectious Disease Control and perform contact tracing. An evaluation of the sexual history 6 months back in time is recommended and standard in most of the country. The county of Västerbotten has one of the lowest incidenses of C. trachomatis in Sweden though the population is younger than the Swedish average. During year 2002 the contact tracing in Västerbotten was evaluated by sending a questionnaire to everyone who reported a case of genital C. trachomatis. We recived 534 (98%) out of 544 questionnaires. The patients reported 1360 partners, 2.5 on average, 1129 were identified. 761 had a known test result and 497 of them were positive, 0.9 on average. 72% of the contact tracers evaluated the sexual history > or = 12 months back in time. 78% of the contact tracings were performed by four social workers. Their patients reported 2.5 partners on average and 80% evaluated > or = 12 months back in time. 14 persons did only one contact tracing, 1.3 partners/index on average, 40% evaluated > or = 12 months back in time. 82% of the partners had sex with the infected patient 0-6 months before the patients was diagnosed with C. trachomatis, 75% out of those with a known test result were infected, 16% had sex 7-12 months before diagnosis, 30% infected. (The C. trachomatis prevalence in Sweden is estimated to be 2.7-4.9% among young women.)'] Is [ Contact tracing a year back worthwhile . Follow the Vasterbottens example to prevent Chlamydia transmission in Sweden ! ]?