Aim and Objectives: Web-based Prostate Specific Antigen (PSA) decision aids are known to promote informed decision-making. There is also some evidence that informed decision-making can result in reduced uptake of PSA testing, thus reducing subsequent costs related to urological intervention, specifically prostate biopsies. The aim of this study was to assess these potential financial benefits. The objectives were: first, to develop a mathematical simulation model based on data from a randomised controlled trial of a web-based PSA decision aid, Prosdex; second, to examine the effect of changes in PSA testing on prostate biopsy numbers and costs.Methods: The simulation model was built using an animated simulation package, Simul8, which allowed for the input of parameter data: 1) Setting; 2) Intention to undertake a PSA test, derived from a RCT of a web-based PSA decision aid; 3) Costs related to PSA tests and prostate biopsies.Results: Total costs varied with changes in the number of PSA tests at a single GP practice, all-Wales and UK level. At the single GP practice level, the effect on costs of changes in PSA testing was minimal. For example, a reduction in PSA testing from 4.6% to 3.6% reduced total costs for the practice by only £1,800. At the UK level, the same reduction in PSA testing lowered costs by approximately £10 million; a relatively small amount of financial resource in the context of a national health budget such as that of the UK National Health Service.Conclusions: The financial impact of web-based PSA decision aids is minimal. The benefit of using PSA decision aids should be viewed in ethical terms and not in financial terms.