Chlamydia trachomatis is the most common cause of bacterial sexually transmitted infections worldwide. Pelvic inflammatory disease, ectopic pregnancy, and infertility in women, and occasionally epididymitis in men, complicate infection. Chlamydia also acts as a marker of increased risk for human immunodeficiency virus infection and may directly contribute to human immunodeficiency virus infection. The prevalence of C. trachomatis has been rising rapidly in most developed countries, including Australia. In New South Wales where this study was conducted, the population rate of C. trachomatis infection doubled between 1996 and 2000, and then doubled again between 2001 and 2005 (national center report).