All chlamydia diagnoses among people accessing sexual health services* in England who are also residents in England, expressed as a rate per 100,000 population. Data is presented by area of patient residence and include those residents in England and those with an unknown residence (data for those residents outside of England is not included).*Sexual health services providing STI related care (Levels 1, 2 or 3). Further details on the levels of sexual healthcare provision are provided in the https://www.bashh.org/about-bashh/publications/standards-for-the-management-of-stis/ .RationaleChlamydia causes avoidable sexual and reproductive ill-health. While chlamydial infections are more commonly found among young adults aged under 25 years, women and men aged 25 years and over are also at-risk of chlamydia.Definition of numeratorThe number of chlamydia diagnoses among people accessing sexual health services in England. Includes those diagnosed through NHS and local authority commissioned testing and excludes those diagnosed through private testing.Chlamydia data is sourced from GUMCAD STI Surveillance System (Level 3) and CTAD Chlamydia Surveillance System (Levels 1 and 2), UKHSA. GUMCAD data is reported by SHSs providing STI related care (Levels 2 or 3). CTAD is reported by laboratories conducting testing for any service (Levels 1, 2 or 3) providing chlamydia testing.A maximum of one chlamydia test or diagnosis per individual is counted within a 6 week period. Any further tests or diagnoses within the 6 week period are not counted. Diagnoses are kept in preference over negative results during this process. Patients cannot be tracked between clinics and therefore de-duplication relies on patient consultations at a single service.The clinical definition used to diagnose chlamydia is given at https://www.bashh.org/guidelines .Definition of denominatorThe denominators for 2012 to 2022 are sourced from Office for National Statistics (ONS) population estimates based on the 2021 Census.Population estimates for 2023 were not available at the time of publication – therefore rates for 2023 are calculated using estimates from 2022 as a proxy.Further details on the ONS census are available from the https://www.ons.gov.uk/census .CaveatsEvery effort is made to ensure accuracy and completeness of GUMCAD data, including web-based reporting with integrated checks on data quality. However, responsibility for the accuracy and completeness of data lies with the reporting service.Data is updated on an annual basis due to clinic or laboratory resubmissions and improvements to data cleaning. Data may differ from previous publications.Figures reported in 2020 and 2021 are notably lower than previous years due to the disruption to SHSs during the national response to the COVID-19 pandemic.