Objective The objective of this study was to estimate the cost and cost-effectiveness of opportunistic screening for Chlamydia trachomatis in Ireland. Methods Prospective cost analysis of an opportunistic screening programme delivered jointly in three types of healthcare facility in Ireland. Incremental costeffectiveness analysis was performed using an existing dynamic modelling framework to compare screening to a control of no organised screening. A healthcare provider perspective was adopted with respect to costs and included the costs of screening and the costs of complications arising from untreated infection. Two outcome measures were examined: major outcomes averted, comprising cases of pelvic inflammatory disease, ectopic pregnancy and tubal factor infertility in women, neonatal conjunctivitis and pneumonia, and epididymitis in men; and quality-adjusted life-years (QALY) gained. Uncertainty was explored using sensitivity analyses and cost-effectiveness acceptability curves. Results The average cost per component of screening was estimated at €26 per offer, €66 per negative case, €152 per positive case and €74 per partner notified and treated. The modelled screening scenario was projected to be more effective and more costly than the control strategy. The incremental cost per major outcomes averted was €6093, and the incremental cost per QALY gained was €94 717. For cost-effectiveness threshold values of €45 000 per QALY gained and lower, the probability of the screening being cost effective was estimated at <1%. Conclusions An opportunistic chlamydia screening programme, as modelled in this study, would be expensive to implement nationally and is unlikely to be judged cost effective by policy makers in Ireland.