OBJECTIVES: To validate an online, freeware, incremental cost-effectiveness calculator that generates and plots results from a decision tree that utilizes Monte Carlo simulations for the variables related to the success probabilities, costs and utilities. METHODS: An online cost-effectiveness calculator and plotter were developed that incorporates a decision tree structure with Monte Carlo simulations to compare two interventions. The online calculator functions on most computer operating systems with JavaScript enabled browsers. The web-based tool creates a scatter plot of incremental costs versus incremental utilities in cost-effectiveness quadrants, and also graphs a cost-effectiveness acceptability curve. This decision analysis calculator is available at www.healthstrategy.com. This online tool uses JavaScript algorithms and was based on an MS Excel model description published by Hughes D. His spreadsheet can be downloaded from www.liv.ac.uk/prg/model.xls. For this validation, the spreadsheet structure was modified slightly to better match the basic online tree. Log-normal and beta distributions are generated from variable inputs that can be modified in the web-based software that cover costs, utilities and probability of both success and failure for two treatment alternatives. RESULTS: Although the min-max range of cost results were similar between the two tools, the range of utility results was greater with the web computations. The MS Excel spreadsheet model (1000 simulations) versus the online calculator (500 simulations) compared as follows: average incremental costs ($1919 vs. $1885), average incremental utilities: (0.058 vs. 0.054) and average incremental cost effectiveness ratio:($33,086 vs. $34,907). CONCLUSIONS: This web-based calculator produced similar results as the MS Excel spreadsheet model. With this online decision tree and Monte Carlo simulator, the user can input their own distribution parameters, then generate and graph incremental costs, incremental utilities and cost effectiveness acceptability curves.