Background Chlamydia trachomatis is the most common sexually transmitted infection in western countries. Most infections are asymptomatic and may cause severe complications. In the Netherlands, 3 years of Chlamydia screening implementation (CSI) have been performed. Here, we model its cost-effectiveness after 10 years. Methods A cost-effectiveness analysis compares the relative costs and effects of two or more scenarios, and is usually expressed as the incremental cost-effectiveness ratio (ICER). The costs of the CSI program included those for hospital care, antibiotics, testing, and productivity loss. We measured the effects as either Major Outcomes Averted (MOA) or Quality Adjusted Life Years (QALY) gained. In the Chlamydia literature, MOAs usually consist of symptomatic pelvic inflammatory disease, chronic pelvic pain (CPP), ectopic pregnancy, infertility, and neonatal pneumonia. We calculated the ICER, the ratio of the above-mentioned costs and effects, for four scenarios: the default screening scenario (annual invitation of all 16–29 year olds), screening for women 16–29 only, for all 16–24 year olds, and biennial screening of all aged 16–29. To account for uncertainty in model parameters, we conducted a probabilistic sensitivity analysis (PSA). Results If we compare the results of the four different scenarios presented in the abstract P5-S7.06 table 1, the default scenario has the most favourable ICER. This is probably due to the fact that the total number of invitations (and thus people tested) per year is the largest in the default scenario, implying that the fixed annual program costs are spread over more tests (and outcomes), which improves the ICER. The cost per QALY of all four scenarios seems acceptable if we include CPP in our QALY estimate. However, the evidence base for CPP forming nearly 90% of all QALYs lost is extremely weak. Therefore, we prefer the cost-per-QALY estimate excluding CPP, which is 30 000–70 000. Considering previous decisions on population screening programs, this ratio is ...