OBJECTIVE: An economic model was developed to estimate the comparative cost-effectiveness of treating unstable bladder (UB) in Belgium with alternative drug treatments and no treatment. The model uses a one-year timeframe and the payer (e.g. Riziv/INAMI, Sickness funds) perspective. METHODS: The study population included patients seeking UB treatment and was divided into successfully treated patients (STP) and patients failing treatment (PFT). The percentage of STP was calculated from clinical efficacy adjusted by annual persistency. For each group of patients, direct and indirect costs were identified, including costs for drugs, incontinence pads, physician visits, lab tests/diagnostics, and associated comorbidities. Resource utilization and costs were obtained from literature, the National Health Insurance System, Official Public Price Ministry Economic Affairs/Pharmaceutical Tarification, clinical trials, and expert medical panels. RESULTS: The prevalence of UB sufferers in Belgium was estimated to be 3% in 2002 (approximately 303,000 people), with only 30% of those patients seeking treatment (approximately 1% of the Belgian population). STP used fewer pads per day, visited physicians less frequently, had fewer lab tests/diagnostics, and experienced fewer comorbidities than PFT. Cost drivers in the model were comorbidities and physician visits. Tolterodine sustained-release (TSR) had comparable efficacy to oxybutynin, but persistence on therapy was higher for TSR. Therefore, effectiveness was higher for TSR versus oxybutynin (37% for TSR, 10% for oxybutynin). Effectiveness was considered 0% for no treatment. Cost per STP was €1,167 for TSR and €2,181 for oxybutynin. Sensitivity analyses varying efficacy and persistency by 10% still resulted in a lower cost per STP for TSR. TSR had an incremental cost-effectiveness of €816. The "no treatment" group demonstrated the highest non-drug costs while providing no efficacy. CONCLUSION: Tolterodine sustained-release is a more cost-effective treatment than oxybutynin in treating UB as measured by cost per successfully treated patient in Belgium.