Objectives: To estimate the average cost per screening offer, cost per testing episode and cost per chlamydia positive episode for an opportunistic chlamydia screening programme (including partner management), and to explore the uncertainty of parameter assumptions, based on the costs to the healthcare system. Methods: A decision tree was constructed and parameterised using empirical data from a chlamydia screening pilot study and other sources. The model was run using baseline data from the pilot, and univariate and multivariate sensitivity analyses were conducted. Results: The total estimated cost for offering screening over 12 months to 33 215 females aged 16-24 was £493 412. The average cost (with partner management) was £14.88 per screening offer (90% credibility interval (CI) 10.34 to 18.56), £21.83 per testing episode (90% CI 18.16 to 24.20), and £38.36 per positive episode (90% CI 33.97 to 42.25). The proportion of individuals accepting screening, the clinician (general practitioner/nurse) time and their relative involvement in discussing screening, the test cost, the time to notify patients of their results, and the receptionist time recruiting patients had the greatest impact on the outcomes in both the univariate and multivariate sensitivity analyses. Conclusions: Results from this costing study may be used to inform resource allocation for current and future chlamydia screening programme implementation.