Chlamydia trachomatis (chlamydia) is the most commonly diagnosed sexually transmitted infection (STI) in England; approximately 70% of diagnoses are in sexually active young adults aged under 25. To facilitate opportunistic chlamydia screening in general practice, a complex intervention, based on a previously successful Chlamydia Intervention Randomised Trial (CIRT), was piloted in England. The modified intervention (3Cs and HIV) aimed to encourage general practice staff to routinely offer chlamydia testing to all 15-24 year olds regardless of the type of consultation. However, when the 3Cs (chlamydia screening, signposting to contraceptive services, free condoms) and HIV was offered to a large number of general practitioner (GP) surgeries across England, chlamydia screening was not significantly increased. This qualitative evaluation addresses the following aims: a) Explore why the modified intervention did not increase screening across all general practices. b) Suggest recommendations for future intervention implementation.