Understanding of the burden of Chlamydia trachomatis infection and its clinical sequelae is hampered by the absence of accurate, well-characterized tests using serological methods to determine past exposure to infection. An "in-house" immunoglobulin G (IgG) enzyme-linked immunosorbent assay (ELISA) based on the C. trachomatis-specific antigen Pgp3 was produced and evaluated against three commercial ELISAs derived from the major outer membrane protein: the Medac pELISA plus, the Savyon SeroCT-IgG ELISA, and the Ani Labsystems IgG enzyme immunoassay. Sensitivities and specificities were determined using sera from both male and female patients (n ‫؍‬ 356) for whom C. trachomatis had been detected in the lower genital tract at least 1 month prior to the testing of the sample and from 722 Chlamydia-negative children aged 2 to 13 years. Chlamydia trachomatis is the commonest sexually transmitted bacterial infection in developed countries, with national surveillance programs consistently showing rising rates of diagnosed infections over the past decade. In the United Kingdom, figures based on cases diagnosed in departments of genitourinary medicine (GUM) suggest a population rate of 190 per 100,000 men and 187 per 100,000 women (52). Reported rates are highly dependent on the level of testing at different clinics, with the probability that many Chlamydia cases are not diagnosed. The population prevalence of uncomplicated genital Chlamydia in 16-to 24-year-olds in the United Kingdom is thought to be between 2% and 6% in both men and women (17,33), while the opportunistic National Chlamydia Screening Programme (2008) indicates a higher prevalence of around 10%, likely due to selective testing of higher-risk individuals (15). Nucleic acid amplification tests, commonly used in GUM clinics, identify infection only when the organism is present. Once infection has been resolved, these tests provide no information on past exposure. While detection rates are rising, due in part to increased screening and testing, the overall prevalence of past C. trachomatis exposure is not known.The prevalence of past exposure to genital C. trachomatis and changes over time in age-specific prevalence can be explored serologically. For instance, in Finland, Lyytikäinen et al. (32) studied pregnant women under the age of 29 using a commercial enzyme-linked immunosorbent assay (ELISA) based on C. trachomatis-specific peptides derived from the major outer membrane protein (MOMP). However, for wider application, confidence in the sensitivity and specificity of available antibody tests is critical (24). None of the current ELISAs have ever been rigorously evaluated against large numbers of well-defined serologically positive and negative control sera; hence, their sensitivity and specificity remain open to question.Chlamydia trachomatis is from the same family, Chlamydiaceae, as Chlamydia pneumoniae, a common respiratory pathogen with which it shares genetic homology (26). Sera from patients exposed to C. trachomatis show diverse serological