OBJECTIVE: Sexually transmitted infection (STI) services were offered by the nongovernmental organization Médecins Sans Frontières-Holland in Banteay Meanchey province, Cambodia, between 1997 and 1999. These services targeted female sex workers but were available to the general population. We conducted an evaluation of the operational performance and costs of this real-life project. METHODS: Effectiveness outcomes (syndromic cure rates of STIs) were obtained by retrospectively analysing patients' records. Annual financial and economic costs were estimated from the provider's perspective. Unit costs for the cost-effectiveness analysis included the cost per visit, per partner treated, and per syndrome treated and cured. FINDINGS: Over 30 months, 11,330 patients attended the clinics; of these, 7776 (69%) were STI index patients and only 1012 (13%) were female sex workers. A total of 15 269 disease episodes and 30 488 visits were recorded. Syndromic cure rates ranged from 39% among female sex workers with genital ulcers to 74% among men with genital discharge; there were variations over time. Combined rates of syndromes classified as cured or improved were around 84-95% for all syndromes. The total economic costs of the project were US 766,046 dollars. The average cost per visit over 30 months was US 25.12 dollars and the cost per partner treated for an STI was US 50.79 dollars. The average cost per STI syndrome treated was US 48.43 dollars, of which US 4.92 dollars was for drug treatment. The costs per syndrome cured or improved ranged from US 46.95-153.00 dollars for men with genital ulcers to US 57.85-251.98 dollars for female sex workers with genital discharge. CONCLUSION: This programme was only partly successful in reaching its intended target population of sex workers and their male partners. Decreasing cure rates among sex workers led to relatively poor cost-effectiveness outcomes overall despite decreasing unit costs.