The prevalence and incidence of genital infections with Chlamydia trachomatis have not yet been determined for broadly based populations within the United States. Largely on the basis of results of prevalence studies from sexually transmitted disease clinics, it has been estimated that C. trachomatis infections are the most common of all sexually transmitted infections and may number 3-5 million cases annually. C. trachomatis isolation rates range from 50% to 80% in certain diseases known to be caused by Chlamydia and from 10% to 30% in sexually active female adolescents and approximately 5% in college women. Cheaper, more-sensitive and widely available diagnostic tests are necessary to adequately define the epidemiology of genital Chlamydia infections in the U.S.